drm/dp: constify DP DPCD helpers

None of the DP DPCD helpers need to modify the DPCD.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/drm_dp_helper.c
Simple merge

diff --cc include/drm/drm_dp_helper.h
Simple merge