drm/amdgpu: remove ctx->lock

KMD reports a warning on holding a lock from drm_syncobj_find_fence,
when running amdgpu_test case “syncobj timeline test”.

ctx->lock was designed to prevent concurrent "amdgpu_ctx_wait_prev_fence"
calls and avoid dead reservation lock from GPU reset. since no reservation
lock is held in latest GPU reset any more, ctx->lock can be simply removed
and concurrent "amdgpu_ctx_wait_prev_fence" call also can be prevented by
PD root bo reservation lock.

call stacks:
=================
//hold lock
amdgpu_cs_ioctl->amdgpu_cs_parser_init->mutex_lock(&parser->ctx->lock);
…
//report warning
amdgpu_cs_dependencies->amdgpu_cs_process_syncobj_timeline_in_dep \
->amdgpu_syncobj_lookup_and_add_to_sync -> drm_syncobj_find_fence \
-> lockdep_assert_none_held_once
…
amdgpu_cs_ioctl->amdgpu_cs_parser_fini->mutex_unlock(&parser->ctx->lock);

Signed-off-by: Ken Xue <Ken.Xue@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c

@@ -230,7 +230,6 @@ static int amdgpu_ctx_init(struct amdgpu_device *adev,
 
 	kref_init(&ctx->refcount);
 	spin_lock_init(&ctx->ring_lock);
-	mutex_init(&ctx->lock);
 
 	ctx->reset_counter = atomic_read(&adev->gpu_reset_counter);
 	ctx->reset_counter_query = ctx->reset_counter;
@@ -352,7 +351,6 @@ static void amdgpu_ctx_fini(struct kref *ref)
 		}
 	}
 	amdgpu_ctx_set_stable_pstate(ctx, AMDGPU_CTX_STABLE_PSTATE_NONE);
-	mutex_destroy(&ctx->lock);
 	kfree(ctx);
 }