drm/amd/display: Add debugfs entry for reading psr state

[Why]
For upcoming PSR stupport it's useful to have debug entry
to verify psr state.

[How]
 - Enable psr dc api for Linux
 - Add psr_state file to eDP connector debugfs
usage e.g.: cat /sys/kernel/debug/dri/0/DP-1/psr_state

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>