drm/i915/bdw: don't try to check IPS state on BDW v2

According to Art, we don't have a way to read back the state reliably at
runtime, through the control reg or the mailbox, at least not without risking
disabling it again.  So drop the readout and checking on BDW.

v2: drop TODO comment (Paulo)
    move POSTING_READ of control reg under HSW branch in disable (Paulo)
    always report IPS as enabled on BDW (Paulo)

References: https://bugs.freedesktop.org/show_bug.cgi?id=71906


Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>