drm/amdgpu: Retire amdgpu_ring.ready flag v4

Start using drm_gpu_scheduler.ready isntead.

v3:
Add helper function to run ring test and set
sched.ready flag status accordingly, clean explicit
sched.ready sets from the IP specific files.

v4: Add kerneldoc and rebase.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>