drm/amdgpu/sdma5: do not execute 0-sized IBs (v2)

This seems to help with https://bugs.freedesktop.org/show_bug.cgi?id=111481

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v2: insert a NOP instead of skipping all 0-sized IBs to avoid breaking older hw

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>