drm/v3d: Introduce Runtime Power Management

drm/v3d: Introduce Runtime Power Management

Commit 90a64adb0876 ("drm/v3d: Get rid of pm code") removed the last
bits of power management code that V3D had, which were actually never
hooked. Therefore, currently, the GPU clock is enabled during probe
and only disabled when removing the driver.

Implement proper power management using the kernel's Runtime PM
framework.

Reviewed-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20260331-v3d-power-management-v9-3-f52ff87bfd36@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>