drm/amd/display: Add dpp_get_gamut_remap functions

We want to be able to read the DPP's gamut remap matrix.

v2:
- code-style and doc comments clean-up (Melissa)

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>