drm/radeon/kms/r600: fix blit support

select the correct max number of bytes per blit based
on whether the size is multiple of 4 bytes.  This
determines whether we can use 8 or 32 bit pixels for
the blit.

airlied: also merged the IB padding patch +
correcting the VS offset for context

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>