drm/ttm: cleanup BO size handling v3

drm/ttm: cleanup BO size handling v3

Based on an idea from Dave, but cleaned up a bit.

We had multiple fields for essentially the same thing.

Now bo->base.size is the original size of the BO in
arbitrary units, usually bytes.

bo->mem.num_pages is the size in number of pages in the
resource domain of bo->mem.mem_type.

v2: use the GEM object size instead of the BO size
v3: fix printks in some places

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com> (v1)
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/406831/