drm/i915: Add engine scratch register to live_lrc_fixed

General purpose registers are per engine and
in a fixed location. Add to live_lrc_fixed.

Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200424214841.28076-1-mika.kuoppala@linux.intel.com