drm/i915/execlists: Remove too early assert

We can't assert that the execlists are active before we set the flag. So
perform the assert after we are expected to have marked the execlists
active.

Fixes: 339ccd35 ("drm/i915: Assert that we always complete a submission to guc/execlists")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180216153210.30551-1-chris@chris-wilson.co.uk