drm/amdgpu: Add init levels

Add init levels to define the level to which device needs to be
initialized.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Tested-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>