drm/nouveau/kms/nv50-: Report possible_crtcs incorrectly on mstos, for now

This commit is seperate from the previous one to make it easier to
revert in the future. Basically, while working on making MSTOs per-head
as opposed to per-head-per-connector I discovered these lovely issues:

https://gitlab.freedesktop.org/xorg/xserver/merge_requests/277
https://gitlab.gnome.org/GNOME/mutter/issues/759

Note as well that Intel already has a temporary workaround for this in
their kernel driver. So, unfortunately we need to follow suit to avoid
causing a regression in userspace. Once these issues get fixed, this
commit should be reverted.
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/dispnv50/disp.c

@@ -2386,6 +2386,18 @@ nv50_display_create(struct drm_device *dev)
 				head->msto = NULL;
 				goto out;
 			}
+
+			/*
+			 * FIXME: This is a hack to workaround the following
+			 * issues:
+			 *
+			 * https://gitlab.gnome.org/GNOME/mutter/issues/759
+			 * https://gitlab.freedesktop.org/xorg/xserver/merge_requests/277
+			 *
+			 * Once these issues are closed, this should be
+			 * removed
+			 */
+			head->msto->encoder.possible_crtcs = crtcs;
 		}
 	}