drm/amd/display: Check returns from drm_dp_dpcd_write

[WHAT & HOW]
drm_dp_dpcd_write() returns negative error on failure and thus returned
values need to be checked.

This fixes 3 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>