drm/amdgpu: initialize num_xcd to 1 for gfx v9_0

Assign value here as the num_xcd is referenced in some gfx9 common path.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>