drm: Don't call drm_for_each_crtc with a non-KMS driver

Fixes oops if userspace calls DRM_IOCTL_GET_CAP for
 DRM_CAP_PAGE_FLIP_TARGET on a non-KMS device node. (Normal userspace
doesn't do that, discovered by syzkaller)
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: f837297a ("drm: Add DRM_MODE_PAGE_FLIP_TARGET_ABSOLUTE/RELATIVE flags v2")
Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161130083002.1520-1-michel@daenzer.net

drivers/gpu/drm/drm_ioctl.c

@@ -254,11 +254,13 @@ static int drm_getcap(struct drm_device *dev, void *data, struct drm_file *file_
 		req->value = dev->mode_config.async_page_flip;
 		break;
 	case DRM_CAP_PAGE_FLIP_TARGET:
+		if (drm_core_check_feature(dev, DRIVER_MODESET)) {
 			req->value = 1;
 			drm_for_each_crtc(crtc, dev) {
 				if (!crtc->funcs->page_flip_target)
 					req->value = 0;
 			}
+		}
 		break;
 	case DRM_CAP_CURSOR_WIDTH:
 		if (dev->mode_config.cursor_width)