drm/i915/execlists: Move the reset bits to a more natural home

In preparation for the next patch, we want the engine to appear idle
after a reset (if there are no requests in flight). For execlists, this
entails clearing the active status on reset, it will be regenerated on
restarting the engine after the reset. In the process, note that a
couple of other status flags and checks could be moved into the
describing function.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205152431.12163-3-chris@chris-wilson.co.uk