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>

drivers/gpu/drm/amd/include/amd_shared.h

@@ -142,6 +142,7 @@ enum PP_FEATURE_MASK {
 
 enum DC_FEATURE_MASK {
 	DC_FBC_MASK = 0x1,
+	DC_MULTI_MON_PP_MCLK_SWITCH_MASK = 0x2,
 };
 
 enum amd_dpm_forced_level;