media: hantro: Fix RK3399 H.264 format advertising

Commit 1f82f2df ("media: hantro: Enable H.264 on Rockchip VDPU2")
enabled H.264 on some SoCs with VDPU2 cores. This had the side-effect
of exposing H.264 coded format as supported on RK3399.

Fix this and clarify how the codec is explicitly disabled on RK3399 on
this driver.

Fixes: 1f82f2df ("media: hantro: Enable H.264 on Rockchip VDPU2")
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Tested-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>