imx-drm: imx-drm-core: merge imx_drm_crtc_register() into imx_drm_add_crtc()

There's no reason for this to be a separate function; merge the
two together.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>