[media] media: ti-vpe: cal: Fix warning: variable dereference before being checked

As reported ctx->sensor is being dereferenced before being checked
in cal_get_external_info(). That being the case it was also checked
twice in multiple other location where v4l2_subdev_call is already
checking it so no need to explicitly check it again.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>