drm/i915/execlists: Record elsp offset during engine setup

Currently, we record the elsp register offset inside init-hw but we only
need to do it once during engine setup (after we know the mmio
iomapping).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180102151235.3949-17-chris@chris-wilson.co.uk