[media] media: davinci: vpif: add check for NULL handler

for da850/omap-l138, there is no need to setup_input_channel_mode()
and set_clock(), to avoid adding dummy code in board file just returning
zero add a check in the driver itself to call the handler only if its
not NULL.

Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>