drm/i915: Reduce locking in execlist command submission

This eliminates six needless spin lock/unlock pairs when writing out
ELSP.

v2: Respin with my preferred colour.
v3: Mostly back to the original colour

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> [v1]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>