media: ti-vpe: vpe: Make sure YUYV is set as default format

media: ti-vpe: vpe: Make sure YUYV is set as default format

v4l2-compliance fails with this message:

   fail: v4l2-test-formats.cpp(672): \
	Video Capture Multiplanar: TRY_FMT(G_FMT) != G_FMT
   fail: v4l2-test-formats.cpp(672): \
	Video Output Multiplanar: TRY_FMT(G_FMT) != G_FMT
	...
   test VIDIOC_TRY_FMT: FAIL

The default pixel format was setup as pointing to a specific offset in
the vpe_formats table assuming it was pointing to the V4L2_PIX_FMT_YUYV
entry. This became false after the addition on the NV21 format (see
above commid-id)

So instead of hard-coding an offset which might change over time we need
to use a lookup helper instead so we know the default will always be what
we intended.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Fixes: 40cc823f7005 ("media: ti-vpe: Add support for NV21 format")
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>