[media] DaVinci-VPIF-Display: Delete an unnecessary variable initialisation in vpif_channel_isr()

The local variable "channel_id" will be reassigned with the following
statement at the beginning. Thus omit the explicit initialisation.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>