drm/i915: placeholder getparam

There are internal patches for a feature which require a parameter to
query whether support exists . These patches cannot be made external
yet. In order to keep existing tests and userspace happy and free from
conflicts, reserve a number for it.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>