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

This patch removes slot->gpu_offset which is not required as
VRAM and PRIV slot are in separate PCI bar.

This patch also removes unused qxl_bo_gpu_offset()

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/372934/