cirrus: stop passing around dst pointers in the blitter

Instead pass around the address (aka offset into vga memory).  Calculate
the pointer in the rop_* functions, after applying the mask to the
address, to make sure the address stays within the valid range.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1489574872-8679-1-git-send-email-kraxel@redhat.com