media: platform: Change the fixed device node number to unfixed value

The driver can be instantiated multiple times, e.g. for a decoder and
an encoder. Moreover, other drivers could coexist on the same system.
This makes the static video node number assignment pointless, so switch
to automatic assignment instead.

Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Xia Jiang <xia.jiang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>