drm/edid: Clear the old cea_rev when there's no CEA extension in the new EDID

It's not a good idea to leave stale cea_rev in the drm_display_info. The
current EDID might not even have a CEA ext block in which case we'd end
up leaving the stale value in place.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1475070703-6435-8-git-send-email-ville.syrjala@linux.intel.com