intel/isl: Fix map between sRGB and linear formats

Not having a solid grasp of these formats, I just feel it is strange that FLT16 formats are not yielded though a rule is there and some SRGB formats just get themselves returned from the generated code. The patch is based on my understanding on what the code tries to achieve.

(Going off for several weeks, so I could reply reviews very late.) CC @gfxstrand, @nchery