drm/amdgpu: retire render backend setup from gfx_v9_4_3

gfx v9_4_3 only support compute. render backend
doesn't need to be involved in any compute shader
execution.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>