drm/display: Make DisplayPort CEC-over-AUX Kconfig name consistent

While most display helpers Kconfig symbols have the DRM_DISPLAY prefix,
the DisplayPort CEC tunnelling implementation uses CONFIG_DRM_DISPLAY_DP_AUX_CEC.

Since the number of users is limited, we can easily rename it to make it
consistent.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20240327-kms-kconfig-helpers-v3-4-eafee11b84b3@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>