drm/i915: Remove save/restore of physical HWS_PGA register

Now that we always restore the HWS registers (both physical and GTT
virtual addresses) when re-initialising the rings, we can eliminate the
superfluous save/restore of the register across suspend and resume.

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