drm/amdgpu: block ring buffer access during GPU recovery

When GPU is in reset, its status isn't stable and ring buffer also need
be reset when resuming. Therefore driver should protect GPU recovery
thread from ring buffer accessed by other threads. Otherwise GPU will
randomly hang during recovery.

v2: correct indent

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>