drm/i915: Mappable_end can't ever be > end

Both DRI1 and DRI2 can never specify a mappable size which goes past the
GTT size.  Don't pretend otherwise.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>