sh: LCDC dcache flush for deferred io

Since writenotify on uncached vmas is unsupported in 2.6.31,
live with cached framebuffer memory in the deferred io
case for now and flush the dcache before forcing refresh.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Acked-by: Magnus damm <damm@igel.co.jp>