drm: Remove duplicate forward declaration

Forward declarations in C are great but I'm pretty sure one is enough.
Signed-off-by: Dawid Kurek <dawikur@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170614213518.GA3554@gmail.com

include/drm/drm_connector.h

@@ -30,8 +30,6 @@
 
 #include <uapi/drm/drm_mode.h>
 
-struct drm_device;
-
 struct drm_connector_helper_funcs;
 struct drm_modeset_acquire_ctx;
 struct drm_device;