imx-drm: imx-drm-core: provide helper function to parse possible crtcs

Provide a helper function to parse possible crtcs before the encoder
is registered.  The crtc mask is derived from the position of the
CRTCs registered in the drm_device.

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>