drm: Don't overwrite UNVERFIED mode status to OK

The way the mode probing works is this:
1. All modes currently on the mode list are marked as UNVERIFIED
2. New modes are on the probed_modes list (they start with
   status OK)
3. Modes are moved from the probed_modes list to the actual
   mode list. If a mode already on the mode list is deemed
   to match one of the probed modes, the duplicate is dropped
   and the mode status updated to OK. After this the
   probed_modes list will be empty.
4. All modes on the mode list are verified to not violate any
   constraints. Any that do are marked as such.
5. Any mode left with a non-OK status is pruned from the list,
   with an appropriate debug message.

What all this means is that any mode on the original list that
didn't have a duplicate on the probed_modes list, should be left
with status UNVERFIED (or previously could have been left with
some other status, but never OK).

I broke that in
commit 05acaec334fc ("drm: Reorganize probed mode validation")
by always assigning something to the mode->status during the validation
step. So any mode from the old list that still passed the validation
would be left on the list with status OK in the end.

Fix this by not doing the basic mode validation unless the mode
already has status OK (meaning it came from the probed_modes list,
or at least a duplicate of it was on that list). This way we will
correctly prune away any mode from the old mode list that didn't
appear on the probed_modes list.

Cc: stable@vger.kernel.org
Cc: Adam Jackson <ajax@redhat.com>
Fixes: 05acaec334fc ("drm: Reorganize probed mode validation")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449177255-9515-2-git-send-email-ville.syrjala@linux.intel.com
Testcase: igt/kms_force_connector_basic/prune-stale-modes
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93332
[danvet: Also applying to drm-misc to avoid too much conflict hell -
there's a big pile of patches from Ville on top of this one.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_probe_helper.c

@@ -233,7 +233,8 @@ static int drm_helper_probe_single_connector_modes_merge_bits(struct drm_connect
 		mode_flags |= DRM_MODE_FLAG_3D_MASK;
 
 	list_for_each_entry(mode, &connector->modes, head) {
-		mode->status = drm_mode_validate_basic(mode);
+		if (mode->status == MODE_OK)
+			mode->status = drm_mode_validate_basic(mode);
 
 		if (mode->status == MODE_OK)
 			mode->status = drm_mode_validate_size(mode, maxX, maxY);