drm/amd/display: document AMDGPU pre-defined transfer functions

Brief documentation about pre-defined transfer function usage on AMD
display driver and standardized EOTFs and inverse EOTFs.

v3:
- Document BT709 OETF (Pekka)
- Fix description of sRGB and pure power funcs (Pekka)

v4:
- Add description of linear and non-linear forms (Harry)

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Co-developed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>