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>