drm/i915: Re-enable the command parser when using PPGTT

In commit

commit 896ab1a5
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Aug 6 15:04:51 2014 +0200

    drm/i915: Fix up checks for aliasing ppgtt

it looks like we accidentally inverted the check that the command
parser should only run when the driver enables some form of PPGTT.

Testcase: igt/gem_exec_parse
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
[danvet: Also drop the comment right above, all production vlv now
have hw ppgtt enabled.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>