drm/i915/bdw: Setup global hardware status page in execlists mode

Write HWS_PGA address even in execlists mode as the global hardware status
page is still required.  This address was previously uninitialized and
HWSP writes would clobber whatever buffer happened to reside at GGTT
address 0.

v2: Break out hardware status page setup into a separate function.

Issue: VIZ-2020
Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>