drm/i915/execlists: Skip resetting RING_CONTEXT_STATUS_PTR

As we now flag when the GPU signals a context-switch and do not read the
status register before we see that signal, we do not have to ensure that
it is cleared upon reset (and can leave it to the GPU to reset it from
the power context).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170201125338.12932-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>