exynos/fimg2d: remove default case from g2d_get_blend_op()

We now validate the blending mode via g2d_validate_mode()
prior to feeding it to g2d_get_blend_op().

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