tests/exynos: remove unused-function build warning

The function g2d_blend_test() is blocked to call because of
feature unsafety. This patch blocks with proper feature name
and also blocks the function itself to remove build warning.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>