drm/xen: Add missing VM_DONTEXPAND flag in mmap callback

With Xen PV Display driver in use the "expected" VM_DONTEXPAND flag
is not set (neither explicitly nor implicitly), so the driver hits
the code path in drm_gem_mmap_obj() which triggers the WARNING.

Signed-off-by: Oleksandr Tyshchenko <[e-mail]>
Reviewed-by: Oleksandr Andrushchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Juergen Gross <[e-mail]>

drivers/gpu/drm/xen/xen_drm_front_gem.c

···
 	 * the whole buffer.
 	 */
 	vma->vm_flags &= ~VM_PFNMAP;
-	vma->vm_flags |= VM_MIXEDMAP;
+	vma->vm_flags |= VM_MIXEDMAP | VM_DONTEXPAND;
 	vma->vm_pgoff = 0;
 
 	/*