drm/amdgpu: use adev->firmware to determine whether to load the PSP module

The per device firmware load method is limited to what makes sense for
that asic rather than whatever arbitrary value may have been set by the
user.

Reviewed-by: Rex Zhu <rezhu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>