drm/amdgpu: Fix a printing message

The printing message "PSP loading VCN firmware" is mis-leading because
people might think driver is loading VCN firmware. Actually when this
message is printed, driver is just preparing some VCN ucode, not loading
VCN firmware yet. The actual VCN firmware loading will be in the PSP block
hw_init. Fix the printing message

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>