drm/amd/display: Enable DPIA trace with DC debug mask

[Why]
DPIA traces from DMUB is not enabled by default, which is
less convenient to debug DPIA related issues because we have
to resort to other means to enable DPIA trace.

[How]
Reuse existing DC debug mask to enable DPIA trace log from
kernel command line. This makes debugging DPIA issues easier
especially when system power state (suspend/reboot) is involved.

To turn on DPIA trace, simply add "amdgpu.dcdebugmask=0x80" to
kernel command line.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>