drm/amd/display: Add a new DC debug mask for PSR-SU

Some issues have been raised that appear to be tied to PSR-SU.
To allow users to confirm they're tied to PSR-SU without turning off
PSR entirely introduce a new debug mask:

amdgpu.dcdebugmask=0x200

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>