drm/amd/display: Optimize gamma calculations

[Why&How]

1. Stack usage is pretty high as fixed31_32 struct is 8 bytes and we
have functions with >30 vars on the stack.

2. Optimize gamma calculation by reducing number of calls to
dc_fixpt_pow Our X points are divided into 32 regions wth 16 pts each.
Each region is 2x the previous, meaning x[i] = 2*x[i-16] for i>=16.
Using (2x)^gamma = 2^gamma * x^gamma, we can recursively compute powers
of gamma, we just need first 16 pts to start it up. dc_fixpt_pow() is
expensive, it computes x^y by doing exp(y*logx) Exp is done by Taylor
series approximation, and log by Newton-like approximation that also
uses exp internally. In short, it's significantly heavier than
run-of-the-mill addition/subtraction/multiply.

Signed-off-by: Krunoslav Kovac <[email protected]>
Reviewed-by: Anthony Koo <[email protected]>
Acked-by: Aric Cyr <[email protected]>
Acked-by: Leo Li <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

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