drm/vmwgfx: don't use ttm bo->offset

Calculate GPU offset within vmwgfx driver itself without depending on
bo->offset.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/372933/


Signed-off-by: Christian König <christian.koenig@amd.com>