drm/i915: Inline feature detection into sanitize_enable_ppgtt

Rather than splitting and hiding away critical parts of
sanitize_enable_ppgtt() into single use macros in the headers, inline
them into the function for clarity.

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