hw/display/artist: Unbreak size mismatch memory accesses

hw/display/artist: Unbreak size mismatch memory accesses

Commit 5d971f9e6725 ("memory: Revert "memory: accept mismatching sizes
in memory_region_access_valid") broke the artist driver in a way that
the dtwm window manager on HP-UX rendered wrong.

Fixes: 5d971f9e6725 ("memory: Revert "memory: accept mismatching sizes in memory_region_access_valid")
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>
(cherry picked from commit e0cf02ce680f11893aca9642e76d6ae68b9375af)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>