cirrus: stop passing around src pointers in the blitter

Does basically the same as "cirrus: stop passing around dst pointers in
the blitter", just for the src pointer instead of the dst pointer.

For the src we have to care about cputovideo blits though and fetch the
data from s->cirrus_bltbuf instead of vga memory.  The cirrus_src*()
helper functions handle that.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1489584487-3489-1-git-send-email-kraxel@redhat.com