gpu: imx: simplify sync polarity setting

Use a function to convert the sync pin to a bit mask for the DI_GENERAL
register, and move this out of the interlace/non-interlace path to the
common path.

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