media: rcar-vin: Create compose rectangle where it is used

The rectangle used to correct the compose settings when changing the
format was created inside a helper function and not where it was used.
This is confusing and makes the code harder to read, fix this.

This cleanup is made possible due to refactoring elsewhere and there is
no functional change.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>