drm/amdgpu: support the port num info based on the capability flag

XGMI TA will set the capability flag to indicate whether the port_num
info is supported or not. KGD checks the flag and accordingly picks up
the right buffer format and send the right command to TA to retrieve
the info.

v2: simplify the code by reusing the same statement (lijo)

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Acked-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>