drm/qxl: don't touch mem.bus.offset

This is internal to TTM and should not be used by drivers directly.

Drop the call to qxl_ttm_io_mem_reserve() and use mem->start instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/389456/