st/va: also honors interlaced preference when providing a video format

st/va: also honors interlaced preference when providing a video format

This fixes a crash when using the prefered video format with vaapisink
on Nvidia hardwares.
Also caught by the following assert:
  nouveau_vp3_video.c:91: Assertion `templat->interlaced' failed.

TEST= gst-launch-1.0 videotestsrc ! video/x-raw, format=NV12 ! vaapisink

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Julien Isorce <j.isorce@samsung.com>
Tested-by: Víctor Manuel Jáquez Leal <vjaquez@igalia.com>
Tested-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>