drm/i915/cmdparser: Add the TIMESTAMP register for the other engines

Since I have been using the BCS_TIMESTAMP to measure latency of
execution upon the blitter ring, allow regular userspace to also read
from that register. They are already allowed RCS_TIMESTAMP!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-32-chris@chris-wilson.co.uk