drm: Fix error path in drm_mode_page_flip_ioctl()

This fixes the error path for platforms that don't define the new
page_flip_target() hook.

Fixes: c229bfbbd04 ("drm: Add page_flip_target CRTC hook v2")
Testcase: igt/kms_flip/basic-flip-vs-dpms
CC: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473269025-16148-1-git-send-email-imre.deak@intel.com

drivers/gpu/drm/drm_crtc.c

@@ -2044,7 +2044,7 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 	}
 
 out:
-	if (ret)
+	if (ret && crtc->funcs->page_flip_target)
 		drm_crtc_vblank_put(crtc);
 	if (fb)
 		drm_framebuffer_unreference(fb);