drm/panfrost: Implement ability to turn on/off GPU clocks in suspend

drm/panfrost: Implement ability to turn on/off GPU clocks in suspend

Currently, the GPU is being internally powered off for runtime suspend
and turned back on for runtime resume through commands sent to it, but
note that the GPU doesn't need to be clocked during the poweroff state,
hence it is possible to save some power on selected platforms.

Add suspend and resume handlers for full system sleep and then add
a new panfrost_gpu_pm enumeration and a pm_features variable in the
panfrost_compatible structure: BIT(GPU_PM_CLK_DIS) will be used to
enable this power saving technique only on SoCs that are able to
safely use it.

Note that this was implemented only for the system sleep case and not
for runtime PM because testing on one of my MediaTek platforms showed
issues when turning on and off clocks aggressively (in PM runtime)
resulting in a full system lockup.

Doing this only for full system sleep never showed issues during my
testing by suspending and resuming the system continuously for more
than 100 cycles.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231109102543.42971-4-angelogioacchino.delregno@collabora.com