media: vicodec: fix wrong sizeimage

The initial sizeimage for the compressed decoder output was wrong.
The size of the output was incorrectly used to calculate the image
size, that should have been the size of the capture.

Rework the code to fix this.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>