drm/i915/execlists: Mark up expected state during reset

Move the BUG_ON around slightly and add some explanations for each to
try and capture the expected state more carefully. We want to compare
the expected active state of our bookkeeping as compared to the tracked
HW state.

References: https://bugs.freedesktop.org/show_bug.cgi?id=111937


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/20191010083242.1387-1-chris@chris-wilson.co.uk