drm/vmwgfx: prevent memory leak in vmw_cmdbuf_res_add

In vmw_cmdbuf_res_add if drm_ht_insert_item fails the allocated memory
for cres should be released.

Fixes: 18e4a466 ("drm/vmwgfx: Fix compat shader namespace")
Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>