exynos: fimg2d: fix comment for G2D_COEFF_MODE_GB_COLOR

The coefficient mode enables use of global color, not alpha.
exynos: fimg2d: unify register style

Register defines all use uppercase hex codes.
exynos: fimg2d: introduce G2D_OP_INTERPOLATE

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