drm/amdgpu: Get rid of amdgpu_job->external_hw_fence

This is a follow-up cleanup to [1]. See bellow refcount balancing
for calling amdgpu_job_submit_direct after this cleanup as far
as I calculated.

amdgpu_fence_emit
	dma_fence_init 1
	dma_fence_get(fence) 2
	rcu_assign_pointer(*ptr, dma_fence_get(fence) 3

---> amdgpu_job_submit_direct completes before fence signaled
			amdgpu_sa_bo_free
				(*sa_bo)->fence = dma_fence_get(fence) 4

			amdgpu_job_free
				dma_fence_put 3

			amdgpu_vcn_enc_get_destroy_msg
				*fence = dma_fence_get(f) 4
				dma_fence_put(f); 3

			amdgpu_vcn_enc_ring_test_ib
				dma_fence_put(fence) 2

			amdgpu_fence_process
				dma_fence_put 1

			amdgpu_sa_bo_remove_locked
				dma_fence_put 0

---> amdgpu_job_submit_direct completes after fence signaled
			amdgpu_fence_process
				dma_fence_put 2

			amdgpu_job_free
				dma_fence_put 1

			amdgpu_vcn_enc_get_destroy_msg
				*fence = dma_fence_get(f) 2
				dma_fence_put(f); 1

			amdgpu_vcn_enc_ring_test_ib
				dma_fence_put(fence) 0

[1] - https://patchwork.kernel.org/project/dri-devel/cover/20220624180955.485440-1-andrey.grodzovsky@amd.com/



Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>