drm/amd/display: Check returned status from core_link_read_dpcd

[WHAT]
The function core_link_read_dpcd returns status which is not used at
all, making them useless assignments.

[HOW]
Print error messages if core_link_read_dpcd does not return DC_OK.

This fixes 2 UNUSED_VALUE issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>