drm/i915: Allow full PPGTT with param override

When PPGTT was disabled by default, the patch also prevented the user
from overriding this behavior via module parameter. Being able to test
this on arbitrary kernels is extremely beneficial to track down the
remaining bugs. The patch that prevented this was:

commit 93a25a9e
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Mar 6 09:40:43 2014 +0100

    drm/i915: Disable full ppgtt by default

By default PPGTT is set to -1. 0 means off, 1 means aliasing only, 2
means full, all other values are reserved.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>