drm/panfrost: Set regulators on/off during system sleep on MediaTek SoCs

drm/panfrost: Set regulators on/off during system sleep on MediaTek SoCs

All of the MediaTek SoCs supported by Panfrost can completely cut power
to the GPU during full system sleep without any user-noticeable delay
in the resume operation, as shown by measurements taken on multiple
MediaTek SoCs (MT8183/86/92/95).

As an example, for MT8195 - a "before" with only runtime PM operations
(so, without turning on/off regulators), and an "after" executing both
the system sleep .resume() handler and .runtime_resume() (so the time
refers to T_Resume + T_Runtime_Resume):

Average Panfrost-only system sleep resume time, before: ~33500ns
Average Panfrost-only system sleep resume time, after:  ~336200ns

Keep in mind that this additional ~308200 nanoseconds delay happens only
in resume from a full system suspend, and not in runtime PM operations,
hence it is acceptable.

Measurements were also taken on MT8186, showing a delay of ~312000 ns.

Testing of this happened on all of the aforementioned MediaTek SoCs, but:
MT8183 got tested only by KernelCI with <=10 suspend/resume cycles
MT8186, MT8192, MT8195 were tested manually with over 100 suspend/resume
cycles with GNOME DE (Mutter + Wayland).

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/20231109102543.42971-7-angelogioacchino.delregno@collabora.com