drm/xe/vm: fix xe_assert()

The region can be used an index into the region_to_mem_type, so we
should be asserting that it is less than the ARRAY_SIZE here.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240318103616.26240-2-matthew.auld@intel.com