drm/i915: Tidy intel_execlists_submission_init

Get to uncore from the engine for better logic organization and use
already available i915 everywhere.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190607084521.16845-2-tvrtko.ursulin@linux.intel.com