drm/vmwgfx: Free hash table allocated by cmdbuf managed res mgr

The hash table created during vmw_cmdbuf_res_man_create was
never freed. This causes memory leak in context creation.
Added the corresponding drm_ht_remove in vmw_cmdbuf_res_man_destroy.

Tested for memory leak by running piglit overnight and kernel
memory is not inflated which earlier was.

Cc: <stable@vger.kernel.org>
Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>