drm/amd/display: add sysfs entry to read PSR residency from firmware

[Why]
Currently there aren't any methods to determine PSR state residency.

[How]
create a sysfs entry for reading residency and internally hook it up
to existing functionality of reading PSR residency from firmware.

[Hamza: dropped the link.h include and made checkpatch happy]

Signed-off-by: Shirish S <shirish.s@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>