drm/i915: clean up PPGTT checking logic

sanitize_enable_ppgtt is the function that checks all the conditions,
honoring a forced ppgtt status or doing auto-detect as necessary.  Just
make sure it returns the right value in all cases and use that in the
macros instead of the confusing intel_enable_ppgtt() function.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[danvet: Don't reenable full ppgtt through the backdoor.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>