exynos: replace G2D_DOUBLE_TO_FIXED macro with function

This also avoids the floating point conversion steps and just
uses pure integer arithmetic.
Since the G2D hardware scaling approach is a bit unintuitive,
document it in the function as well.

v2: Explicitly mention the normalization constant.
v3: Use common commenting style as pointed out by
    Emil Velikov <emil.l.velikov@gmail.com>.

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>