drm/amd/display: Add debugfs entry for LTTPR register status

[Why]
Functionality of LTTPR is reporter through the DPCD register

[How]
Expose a interface in debugfs to read the current status of
LTTPR as reported from the device's DPCD register

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>