drm/edid: abstract debugfs override EDID show better

Add a function to dump the override EDID in debugfs. This hides the
override EDID management better in drm_edid.c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/74defa7b595f51e6c1f2eacd9c799d567d29f053.1666614699.git.jani.nikula@intel.com