[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.