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>