imx-drm: imx-drm-core: improve safety of imx_drm_add_crtc()

We must not add more CRTCs than we have declared to the vblank
helpers, otherwise we overflow their arrays.  Force failure if we
exceed the number of CRTCs.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>