drm/i915: Restrict the aliasing-ppgtt to the size of the ggtt

The aliasing-ppgtt is not allowed to be smaller than the ggtt, nor
should we advertise it as being any bigger, or else we may get sued for
false advertisement.

Testcase: igt/gem_exec_big
Fixes: 0b718ba1 ("drm/i915/gtt: Downgrade Cherryview back to aliasing-ppgtt")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190902040303.14195-1-chris@chris-wilson.co.uk