drm/edid: keep track of alloc size in drm_do_get_edid()

We'll want to return the allocated buffer size in the future. Keep track
of it.

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/8e4261d8c2947ea99240ea929f09a04878235f4e.1654674560.git.jani.nikula@intel.com