drm/i915: Add TIMESTAMP to register whitelist

This is needed for the Mesa Vulkan driver on Haswell.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Kristian Høgsberg <krh@bitplanet.net>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1457335830-30923-2-git-send-email-jordan.l.justen@intel.com