drm/bridge: remove ->get_edid callback

There are no more users of the ->get_edid callback left. They've all
been converted to ->edid_read. Remove the callback, and the fallback in
drm_bridge_edid_read().

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/34407a355ec6848fc44f8c30d245fcbc5687195e.1706038510.git.jani.nikula@intel.com