drm: stop vmgfx driver explosion

If you do a page flip with no flags set then event is NULL. If event is
NULL then the vmw_gfx driver likes to go digging into NULL and extracts
NULL->base.file_priv.

On a modern kernel with NULL mapping protection it's just another oops,
without it there are some "intriguing" possibilities.

What it should do is an open question but that for the driver owners to
sort out.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>