drm/i915/edp: Add WaKVMNotificationOnConfigChange:bdw

According to bspec this workaround helps to reduce lag and improve
performance on edp.

Documentation suggests this for bdw and all gen9. However evidence
shows that this register is missing on gen9 and causing unclaimed mmio
access if we access it. So apply to bdw only where the reg
exists and can hold its value.

v2: drop skl

References: HSD#2134579
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465309159-30531-11-git-send-email-mika.kuoppala@intel.com