vmwgfx: return -EFAULT instead of number of bytes remaining

The intent here was to return an error code, but instead the code
returns the number of bytes remaining (that weren't copied).

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>