drm/amdgpu: add DP audio support for si dce6 (v3)

v2: refine dce_v6_0_audio_endpt_wreg() and unify inconsistent method names
v3: fix num_pins for tahiti, pitcairn, verde and oland

Signed-off-by: Xiaojie Yuan <Xiaojie.Yuan@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>