drm/amd/display: Add pwait status to DMCUB diagnostics

[Why]
To know if the firmware is idle when logging.

[How]
Add the pwait status to the DMCUB diagnostics.

Reviewed-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>