drm/edid: split out invalid block filtering to a separate function

It's such a special case there's no point in keeping it inline in the
happy day scenario, confusing matters.

v2: Rebase on the invalid block filtering fix

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/ea8ccd654b5beca6ccf99666754aea09c836b6a5.1648752228.git.jani.nikula@intel.com