drm/amdgpu: added AMD GPU instance counting V2

Count all GPU instances from AMD(including iGPUs and
dGPUs) in the system.

V2: drop unnecessary initialization for other gpu_info
    members except mutex

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>