drm/radeon/kms: set SX_MISC in the r6xx blit code (v2)

Mesa may set it to 1, causing all primitives to be killed.

v2: also update the r7xx code

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>