cirrus: fix cirrus_invalidate_region

off_cur_end is exclusive, so off_cur_end == cirrus_addr_mask is valid.
Fix calculation to make sure to allow that, otherwise the assert added
by commit f153b563 can trigger for valid
blits.

Test case: boot windows nt 4.0

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1489579606-26020-1-git-send-email-kraxel@redhat.com
(cherry picked from commit e048dac6)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>