drm/i915: Mark elsps submitted when they are pushed to hw

Now when we have requests this deep on call chain, we can mark
the elsp being submitted when it actually is. Remove temp variable
and readjust commenting to more closely fit to the code.

v2: Avoid tmp variable and reduce number of writes (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>