drm/loongson/7a2000: convert to struct drm_edid

Prefer the struct drm_edid based functions for reading the EDID and
updating the connector.

Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://patchwork.freedesktop.org/patch/msgid/b8c477279a6ae1b44df72bee230180e277087fae.1715691257.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>