drm/i915: Document that mmap forwarding is discouraged

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>