imx-drm: imx-drm-core: Remove unneeded forward declaration

There is no need to forward declare 'struct imx_drm_crtc' because the
"imx-drm.h" header file already includes it.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/imx-drm/imx-drm-core.c

@@ -29,8 +29,6 @@
 
 #define MAX_CRTC	4
 
-struct imx_drm_crtc;
-
 struct imx_drm_component {
 	struct device_node *of_node;
 	struct list_head list;