drm/i915: Update execlists context descriptor format commentary

The comments describing the Context Descriptor Format are off by a bit
for the size of the context ID.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-16-git-send-email-chris@chris-wilson.co.uk