drm/display: dp-cec: use new DCPD access helpers

Switch drm_dp_cec.c to use new set of DPCD read / write helpers.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20250324-drm-rework-dpcd-access-v4-4-e80ff89593df@oss.qualcomm.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>