drm/amd/amdgpu: fixing read wrong pf2vf data in SRIOV

[Why]
This fixes 892deb48 ("drm/amdgpu: Separate vf2pf work item init from virt data exchange").
we should read pf2vf data based at mman.fw_vram_usage_va after gmc
sw_init. commit 892deb48 breaks this logic.

[How]
calling amdgpu_virt_exchange_data in amdgpu_virt_init_data_exchange to
set the right base in the right sequence.

v2:
call amdgpu_virt_init_data_exchange after gmc sw_init to make data
exchange workqueue run

v3:
clean up the code logic

v4:
add some comment and make the code more readable

Fixes: 892deb48

 ("drm/amdgpu: Separate vf2pf work item init from virt data exchange")
Signed-off-by: Jingwen Chen <Jingwen.Chen2@amd.com>
Reviewed-by: Horace Chen <horace.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>