drm/bridge: megachips: switch to ->edid_read callback

Prefer using the struct drm_edid based callback and functions.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/f1e3fa51de1625f9f361b7cdb81fd74d51fb9dba.1706038510.git.jani.nikula@intel.com