drm/i915/execlists: HWACK checking superseded checking port[0].count

The HWACK bit more generically solves the problem of resubmitting ESLP
while the hardware is still processing the current ELSP write. We no
longer need to check port[0].count itself.

References: ba74cb10

 ("drm/i915/execlists: Delay writing to ELSP until HW has processed the previous write")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517115647.17205-1-chris@chris-wilson.co.uk