drm/amd/pp: Workaround flickering issue on RV

Screen flickering observed while running 1080p video using
MPV_VAAPI/VDPAU with 4x4K@60 monitors

Need to set higher mclk in this configuration.

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>