drm/amd/display: Add debugfs entry for ILR

[Why & How]
In order to know the intermediate link rates supported by the eDP
panel and test to select the optimized link rate to save power,
create a new debugfs entry "ilr_setting" for
setting ILR.

Reviewed-by: Aurabindo Jayamohanan Pillai <Aurabindo.Pillai@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>