drm/i915: Fixup pfit disabling for gen2/3

The recent rework of the pfit handling didn't take into account that
the panel fitter is fixed to pipe B:

commit 24a1f16d
Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Fri Feb 8 16:35:37 2013 +0200

    drm/i915: disable shared panel fitter for pipe

Fix this up by properly computing the pipe the pfit is on. Also
extract the logic into its own function, add a debug assert to check
that the pipe is off (mostly just documentation) and add some debug
output.

If pipe A was disabled after pipe B was set up, the panel fitter will
be disabled. Now most userspace doesn't do modesets in this order,
which is why I couldn't ever reproduce this and why it took me so long
to figure out.

We really need hw state readout and check support for the pannel
fitter ...

Reported-by: Hans de Bruin <jmdebruin@xmsnet.nl>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Hans de Bruin <jmdebruin@xmsnet.nl>
References: http://permalink.gmane.org/gmane.comp.freedesktop.xorg.drivers.intel/19049


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>