drm: Rename MODE_UNVERIFIED to MODE_STALE

MODE_UNVERIFIED actually means that the mode came from a previous probe,
and if the new probe doesn't produce a matching mode it will get pruned
from the list. Rename the flag to MODE_STALE to better convey the
meaning.

v2: Rebased due to conflicts with Daniel's doc stuff

Cc: Adam Jackson <ajax@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449779948-10906-1-git-send-email-ville.syrjala@linux.intel.com