drm/amd/pp: Expose set/get_power_limit for DGPU

User can change power limit between
[0, 1] * max power limit.

Set power limit to 0, restore to max power limit.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>