Too many new drm driver writers seem to look at i915 for inspiration.
But we have two ways to do mmap, so discourage readers from the old,
ugly version. In a new driver we'd just expose two mmap offsets per
object, one for the gtt map and the other for the cpu map.

v2: Make it clear that i915 does cpu mmaps this way for past
cluelessness^W^W historical reasons. Asked for by Jani.

Cc: "Cheng, Yao" <yao.cheng@intel.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>