media: ti-vpe: vpe: fix a v4l2-compliance warning about invalid pixel format

v4l2-compliance warns with this message:

   warn: v4l2-test-formats.cpp(717): \
 	TRY_FMT cannot handle an invalid pixelformat.
   warn: v4l2-test-formats.cpp(718): \
 	This may or may not be a problem. For more information see:
   warn: v4l2-test-formats.cpp(719): \
 	http://www.mail-archive.com/linux-media@vger.kernel.org/msg56550.html


	...
   test VIDIOC_TRY_FMT: FAIL

We need to make sure that the returns a valid pixel format in all
instance. Based on the v4l2 framework convention drivers must return a
valid pixel format when the requested pixel format is either invalid or
not supported.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
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>