drm/amdgpu: Fix DEBUG_LOCKS_WARN_ON(depth <= 0) in amdgpu_ctx.lock

If CS is submitted using guilty ctx, we terminate amdgpu_cs_parser_init
before locking ctx->lock, latter in amdgpu_cs_parser_fini we still are
trying to release the lock just becase parser->ctx != NULL.

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>