gpu: host1x: Drop excess kernel-doc entry @key

Fix kernel-doc warning in host1x:

../drivers/gpu/host1x/bus.c:774: warning: Excess function parameter 'key' description in '__host1x_client_register'

Fixes: 0cfe5a6e

 ("gpu: host1x: Split up client initalization and registration")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-tegra@vger.kernel.org
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>