drm/panfrost: Mark simple_ondemand governor as softdep

Panfrost DRM driver uses devfreq to perform DVFS, while using simple_ondemand
devfreq governor by default.  This causes driver initialization to fail on
boot when simple_ondemand governor isn't built into the kernel statically,
as a result of the missing module dependency and, consequently, the required
governor module not being included in the initial ramdisk.  Thus, let's mark
simple_ondemand governor as a softdep for Panfrost, to have its kernel module
included in the initial ramdisk.

This is a rather longstanding issue that has forced distributions to build
devfreq governors statically into their kernels, [1][2] or has forced users
to introduce some unnecessary workarounds. [3]

For future reference, not having support for the simple_ondemand governor in
the initial ramdisk produces errors in the kernel log similar to these below,
which were taken from a Pine64 RockPro64:

  panfrost ff9a0000.gpu: [drm:panfrost_devfreq_init [panfrost]] *ERROR* Couldn't initialize GPU devfreq
  panfrost ff9a0000.gpu: Fatal error during GPU init
  panfrost: probe of ff9a0000.gpu failed with error -22

Having simple_ondemand marked as a softdep for Panfrost may not resolve this
issue for all Linux distributions.  In particular, it will remain unresolved
for the distributions whose utilities for the initial ramdisk generation do
not handle the available softdep information [4] properly yet.  However, some
Linux distributions already handle softdeps properly while generating their
initial ramdisks, [5] and this is a prerequisite step in the right direction
for the distributions that don't handle them properly yet.

[1] https://gitlab.manjaro.org/manjaro-arm/packages/core/linux/-/blob/linux61/config?ref_type=heads#L8180
[2] https://salsa.debian.org/kernel-team/linux/-/merge_requests/1066
[3] https://forum.pine64.org/showthread.php?tid=15458
[4] https://git.kernel.org/pub/scm/utils/kernel/kmod/kmod.git/commit/?id=49d8e0b59052999de577ab732b719cfbeb89504d
[5] 97ac4d37aa

Cc: Diederik de Haas <didi.debian@cknow.org>
Cc: Furkan Kardame <f.kardame@manjaro.org>
Cc: stable@vger.kernel.org
Fixes: f3ba91228e ("drm/panfrost: Add initial panfrost driver")
Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4e1e00422a14db4e2a80870afb704405da16fd1b.1718655077.git.dsimic@manjaro.org

drivers/gpu/drm/panfrost/panfrost_drv.c

@@ -838,3 +838,4 @@ module_platform_driver(panfrost_driver);
 MODULE_AUTHOR("Panfrost Project Developers");
 MODULE_DESCRIPTION("Panfrost DRM Driver");
 MODULE_LICENSE("GPL v2");
+MODULE_SOFTDEP("pre: governor_simpleondemand");