imx-drm: imx-drm-core: Staticize imx_drm_find_crtc()

Fix the following static checker warning:

drivers/staging/imx-drm/imx-drm-core.c:89:21: warning: symbol 'imx_drm_find_crtc' was not declared. Should it be static?
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
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

@@ -86,7 +86,7 @@ static int imx_drm_driver_unload(struct drm_device *drm)
 	return 0;
 }
 
-struct imx_drm_crtc *imx_drm_find_crtc(struct drm_crtc *crtc)
+static struct imx_drm_crtc *imx_drm_find_crtc(struct drm_crtc *crtc)
 {
 	struct imx_drm_device *imxdrm = crtc->dev->dev_private;
 	unsigned i;