drm/i915/gt: Small tidy of gen8+ breadcrumb emission

Use a local to shrink a line under 80 columns, and refactor the common
emit_xcs_breadcrumb() wrapper of ggtt-write.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200504180507.6017-1-chris@chris-wilson.co.uk