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: NEvan Quan <evan.quan@amd.com>
Reviewed-by: NNicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>