drm: Make drm_crtc_helper.h standalone includible

The file refers to a bunch of structure declared in drm_crtc.h, so
include it to make sure the drm_crtc_helper.h header can be included
standalone.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_crtc_helper.h

@@ -39,6 +39,8 @@
 
 #include <linux/fb.h>
 
+#include <drm/drm_crtc.h>
+
 enum mode_set_atomic {
 	LEAVE_ATOMIC_MODE_SET,
 	ENTER_ATOMIC_MODE_SET,