exynos: fimg2d: introduce G2D_OP_INTERPOLATE

This sets up the blending equation in the following way:
out = src * src_alpha + dst * (1 - src_alpha)

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Joonyoung Shim <jy0922.shim@samsung.com>