drm/bochs: use drm_gem_vram_offset to get bo offset v2

Switch over to GEM VRAM's implementation to retrieve 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/372931/