drm/amd/display: fix read errors pertaining to dp_lttpr_status_show()

Currently, it is likely that we will read the relevant LTTPR caps after
link training has completed (which can cause garbage data to be read),
however according to the DP 2.0 spec that should be done before link
training has commenced. So, instead of reading the registers on demand,
use the values provided to us by DC.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>