drm/amdgpu: cleanup ttm debug sdma vram access function

Some suggested cleanups to declutter ttm when doing debug VRAM access over
SDMA.

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