drm/amdgpu: Power up UVD 3 for FW validation (v2)

Unlike later versions, UVD 3 has firmware validation.
For this to work, the UVD should be powered up correctly.

When DPM is enabled and the display clock is off,
the SMU may choose a power state which doesn't power
the UVD, which can result in failure to initialize UVD.

v2:
Add code comments to explain about the UVD power state
and how UVD clock is turned on/off.

Fixes: b38f3e80 ("drm amdgpu: SI UVD v3_1 (v2)")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>