drm/i915: don't save/restore HWS_PGA reg for kms

We already do that as part of the ringbuffer re-setup at resume time.
Furthermore the register offset has moved on gen6+ around quite a bit,
and on ilk/gm45 we also need to restore the HWS reg for the bsd ring,
not just the render ring.

So again in kms mode this is only confusing a best, hence don't
bother.

v2: Fixup logic, noticed by Paulo Zanoni.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>