Add @cache description to eliminate a kernel-doc warning.

include/linux/host1x.h:104: warning: Function parameter or member 'cache' not described in 'host1x_client'

Fixes: 1f39b1df ("drm/tegra: Implement buffer object cache")
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: linux-tegra@vger.kernel.org
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: NThierry Reding <treding@nvidia.com>