drm/vmwgfx: Fix buffer object eviction

Commit 19be5570

 ("drm/ttm: add operation ctx to ttm_bo_validate v2")
introduced a regression where the vmwgfx driver refused to evict a
buffer that was still busy instead of waiting for it to become idle.

Fix this.

Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>