drm/amdgpu: implement querying ras error count for mmhub9.4

Get mmhub error counter by accessing EDC_CNT registers.

v2: Add mmhub_v9_4_ prefix for local static variable and function

Signed-off-by: Dennis Li <dennis.li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>