drm/mediatek/dp: 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/3d783478e25e71f12f66c2caedb1f9205d4d8a44.1706038510.git.jani.nikula@intel.com