drm/i915/execlists: Tidy enabling execlists

Move the register settings for enabling execlists into its own function
for clarity.

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/20180102151235.3949-18-chris@chris-wilson.co.uk