tests/exynos: use XRGB8888 for framebuffer

This matches the G2D color mode that is used in the entire code.
The previous (incorrect) RGBA8888 would only work since the
Exynos mixer did its configuration based on the bpp, and not
based on the actual pixelformat.

Reviewed-by: Hyungwon Hwang <human.hwang@samsung.com>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>