radeonfb: fix problem with color expansion & alignment

The engine on some radeon variants locks up if color expansion is called
for non aligned source data.  This patch enables a feature of the core
fbdev to request aligned input pixmaps and uses the HW clipping engine to
clip the output to the requested size

Addresses http://bugzilla.kernel.org/show_bug.cgi?id=11875

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Tested-by: James Cloos <cloos@jhcloos.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>