tests/exynos: remove struct fimg2d_test_case

It doesn't make sense to keep this structure, since we
can just call all tests directly. An inspection of the
git history shows that no code ever used this
abstraction in the past.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>