drm/i915: Documentation for multiple GGTT views

A short section describing background, implementation and intended usage.

v2:
    * Align section name between template and DOC comment. (Michel Thierry)

For: VIZ-4544
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>