drm/i915: rip out dev_priv->has_gem

Always true these days. It has been added originally to work
around some issues with the agp layer in 2.6.29:

commit ac5c4e76
Author: Dave Airlie <airlied@redhat.com>
Date:   Fri Dec 19 15:38:34 2008 +1000

    drm/i915: GEM on PAE has problems - disable it for now.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>