drm/amdgpu: clean up the superfluous space and align the comment text for amdgpu_ttm

This patch cleans up spaces and align the text to refine the comment for
amdgpu_ttm.

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