drm: Add some missing forward declarations

The drm_file and drm_clip_rect structures are used throughout the file
but they are never declared nor pulled in through an include. Add
forward declarations to make them available.
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

include/drm/drm_crtc.h

@@ -38,7 +38,8 @@ struct drm_device;
 struct drm_mode_set;
 struct drm_framebuffer;
 struct drm_object_properties;
-
+struct drm_file;
+struct drm_clip_rect;
 
 #define DRM_MODE_OBJECT_CRTC 0xcccccccc
 #define DRM_MODE_OBJECT_CONNECTOR 0xc0c0c0c0