Initialize flags member of struct drm_915_gem_mmap as the upcoming inteldrm(4)

update checks it.

Upstream fixed this differently, but that fix doesn't work for us as our
ioctl(2) subsystem works differently from Linux.

ok jsg@, tom@

driver/xf86-video-intel/src/sna/kgem.c

@@ -6019,6 +6019,7 @@ retry:
 	mmap_arg.handle = bo->handle;
 	mmap_arg.offset = 0;
 	mmap_arg.size = bytes(bo);
+	mmap_arg.flags = 0;
 	if ((err = do_ioctl(kgem->fd, DRM_IOCTL_I915_GEM_MMAP, &mmap_arg))) {
 		assert(err != EINVAL);