drm/vmwgfx: Don't access file_priv in cursor_set when handle==0

We want to disable the cursor by calling ->cursor_set() with handle=0
from places where we don't have a file_priv, so don't try to access it
unless necessary.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>