drm/edid: move drm_edid_is_zero to top, make edid argument const

drm_edid_is_zero will be used by drm_edid_block valid, move it up.
raw_edid argument can be const.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>