media: vivid: check if the cec_adapter is valid

If CEC is not enabled for the vivid driver, then the adap pointer is NULL
and 'adap->phys_addr' will fail.

Cc: <stable@vger.kernel.org>      # for v4.12 and up
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>