drm/amdgpu: expose benchmarks via debugfs

They provide a nice smoke test of transfer performance
using SDMA.  Allow the user to run these at runtime
rather than only at init time.

v2: fix permissions (Alex)

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