imx-drm: imx-hdmi: provide register modification function

There are a load of read-modify-write patterns to change bitfields in
various registers in this driver; provide a helper to perform this
manipulation.
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>