tests/exynos: fimg2d: add a checkerboard test

This makes it easier to spot memory corruptions which don't become
visible when using a plain buffer filled with a solid color (so
corruptions that are just a permutation of the bytes in the buffer).

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Inki Dae <inki.dae@samsung.com>
Tested-by: Joonyoung Shim <jy0922.shim@samsung.com>