vebox: fix indication of field ordering in sequence.

vebox: fix indication of field ordering in sequence.

VEBOX_DNDI_STATE::dndi_top_first indicates whether the top field is
first in sequence (TFF), or if the bottom field comes first. This is
an indication of the temporal sequence for input frames, available
in the "history" buffer. As such, the correct flag to check against,
from a VA-API perspective, is VA_DEINTERLACING_BOTTOM_FIELD_FIRST.

https://bugs.freedesktop.org/show_bug.cgi?id=72518
https://bugs.freedesktop.org/show_bug.cgi?id=72522

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>