Patch from Sascha Hauer

The dmacr needs different settings on some boards. This patch makes the
register configurable by the platform part.
Also we have imxfb_disable_controller(), so lets use it.

Signed-off-by: Steven Scholz
Signed-off-by: Sascha Hauer
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>