drm/amdgpu/benchmark: use dev_info rather than DRM macros for logging

So we can tell which output goes to which device when multiple GPUs
are present.  Also while we are here, convert DRM_ERROR to dev_info.
The error cases are not critical.

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