drm/amdgpu/vg20: Change the load type of vega20 to psp (v2)

Modified the vega20 load type to psp now that psp
support is implemented.

v2: squash in fixes history (Alex)

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>