drm/vmwgfx: Fix a buffer object leak

A buffer object leak was introduced when fixing a premature buffer
object release. Fix this.

Cc: <stable@vger.kernel.org>
Fixes: 73a88250

 ("Fix a destoy-while-held mutex problem.")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>