S3C2410: ensure that FB_BLANK_POWERDOWN shuts down the controller

When a blank level of FB_BLANK_POWERDOWN is used, we should shut down the
controller so that it no longer tries to produce any panel signals or
data, and shuts down the DMA which is not needed.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Cc: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>