drm/amd/display: add get_gamut_remap helper for MPC3

We want to be able to read the MPC's gamut remap matrix similar to
what we do with .dpp_get_gamut_remap functions. On the other hand, we
don't need a hook here because only DCN3+ has the MPC gamut remap
block, being absent in previous families.

Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>