drm/amdgpu: Remove FW_LOAD_DIRECT type support on VI

AMDGPU_FW_LOAD_DIRECT is used for bring up.
Now it don't work any more. so remove the support.

v2: Add warning message if user select
   AMDGPU_FW_LOAD_DIRECT/AMDGPU_FW_LOAD_PSP on VI.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>