cirrus: allow zero source pitch in pattern fill rops

The rops used by cirrus_bitblt_common_patterncopy only use
the destination pitch, so the source pitch shoul allowed to
be zero and the blit with used for the range check around the
source address.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Message-id: 1485272138-23249-1-git-send-email-w.bumiller@proxmox.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
