drm/amdgpu/gfx: set sched.ready status after ring/IB test in gfx

sched.ready is nothing with ring initialization, it needs to set
to be true after ring/IB test in amdgpu_ring_test_helper to tell
the ring is ready for submission.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>