drm/i915: Correct PPGTT total size

Our code allows have a PPGTT that is smaller than the maximum size for
GEN6-GEN7. Though I don't think this actually ever occurs, the code may
as well work properly and more importantly look correct by using the
variable size instead of the HW max.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>