drm/vram-helper: don't use ttm bo->offset v4

Calculate GEM VRAM bo's offset within vram-helper without depending on
bo->offset.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/372938/