drm/i915: Provide DPIO diagrams as docboox tables

The ascii art version of the DPIO diagram gets mangled by docbook, so
we can't use it there. Insted provide another version built using
<table>.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chon Ming Lee <chon.ming.lee@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>