amdgpu/vce3: Simplify vce_v3_0_hw_init and ensure both rings default to not ready.

Simplified the ring test and added logic to ensure rings are marked not ready
by default.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>