drm/amdgpu/display: add flag for multi-display mclk switching

Add a dcfeaturemask flag for mclk switching.  Disable by default;
enable once the feature has seen more testing.

Set amdgpu.dcfeaturemask=2 on the kernel command line in grub
to enable this.

Acked-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>