drm/amdgpu/display: add dc feature mask for psr enablement

[Why]
Adding psr mask to dc features allows selectively disable/enable psr.
Current psr implementation may not work with non-pageflipping application.
Until resolved it should be disabled by default.

[How]
Add dcfeaturemask for psr enablement. Disable by default.
To enable set amdgpu.dcfeaturemask=0x8 in grub kernel command line.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>