drm/i915/gtt: Don't check PPGTT presence on PPGTT-only platforms

We missed one place where we check PPGTT-only platform for PPGTT
presence. Let's remove it.
While I'm here let's assert that this particular code is never called on
pre-gen8 platforms.

References: 4bdafb9d ("drm/i915: Remove i915.enable_ppgtt override")
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190702113149.21200-2-michal.winiarski@intel.com