[media] v4l: ti-vpe: zero out reserved fields in try_fmt

Zero out the reserved formats in v4l2_pix_format_mplane and
v4l2_plane_pix_format members of the returned v4l2_format pointer when passed
through TRY_FMT ioctl.

This ensures that the user doesn't interpret the non-zero fields as some data
passed by the driver, and ensures compliance.

Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>