drm/i915: Add a brief description of the VLV display PHY internals

Document the internal structure of the VLV display PHY a bit to help
people understand how the different register blocks relate to each
other.

v2: Add a bit more text
    Make it a DOC: comment, but leave the ascii art out since
    it would get mangled

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