drm/amd/display: add a debugfs interface for the DMUB trace mask

For features that are implemented primarily in DMUB (e.g. PSR), it is
useful to be able to trace them at a DMUB level from the kernel,
especially when debugging issues. So, introduce a debugfs interface that
is able to read and set the DMUB trace mask dynamically at runtime and
document how to use it.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>