drm/vmwgfx: Fix handling of errors returned by 'vmw_cotable_alloc()'

'vmw_cotable_alloc()' returns an error pointer on error, not NULL.
Propagate the error code, instead of returning -ENOMEM unconditionally

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Sinclair Yeh <syeh@vmware.com>