drm: Use register writes instead of BITBLT_MULTI packets for buffer swap blits

This takes up two more ring buffer entries per rectangle blitted but makes sure
the blit is performed top to bottom, reducing the likelyhood of tearing.

Signed-off-by: Dave Airlie <airlied@linux.ie>

diff --cc drivers/char/drm/radeon_drv.h
Simple merge

diff --cc drivers/char/drm/radeon_state.c
Simple merge