tests/exynos: handle G2D_IMGBUF_COLOR in switch statements

This fixes a compiler warning about missing handling of enum
values in the switch statements.

Also remove the silent mapping to G2D_IMGBUF_GEM when an
unknown buffer type is encountered. We have full control
about the type here, and if it's unknown then we obviously
have a bug in the code.

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