drm/vmwgfx: return -EFAULT for copy_to_user errors

copy_to/from_user() returns the number of bytes remaining to be copied
but we want to return a negative error code here.  This gets returned to
userspace.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>