drm/vmwgfx: Fix error printout on signals pending

The function vmw_master_check() might return -ERESTARTSYS if there is a
signal pending, indicating that the IOCTL should be rerun, potentially from
user-space. At that point we shouldn't print out an error message since that
is not an error condition. In short, avoid bloating the kernel log when a
process refuses to die on SIGTERM.

Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>