drm/amd/amdgpu/si_dma: Fix some function name disparity

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/si_dma.c:320: warning: expecting prototype for cik_dma_vm_copy_pte(). Prototype was for si_dma_vm_copy_pte() instead
 drivers/gpu/drm/amd/amdgpu/si_dma.c:412: warning: expecting prototype for si_dma_pad_ib(). Prototype was for si_dma_ring_pad_ib() instead
 drivers/gpu/drm/amd/amdgpu/si_dma.c:425: warning: expecting prototype for cik_sdma_ring_emit_pipeline_sync(). Prototype was for si_dma_ring_emit_pipeline_sync() instead

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>