drm/i915: Add I915_PARAM_MMAP_GTT_VERSION to advertise unlimited mmaps

Now that we have working partial VMA and faulting support for all
objects, including fence support, advertise to userspace that it can
take advantage of unlimited GGTT mmaps.

v2: Make room in the kerneldoc for a more detailed explanation of the
limitations of the GTT mmap interface.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160825180519.11341-1-chris@chris-wilson.co.uk