drm/i915: don't enable PM_VEBOX_CS_ERROR_INTERRUPT

drm/i915: don't enable PM_VEBOX_CS_ERROR_INTERRUPT

The code to handle it is broken - there's simply no code to clear CS
parser errors on gen5+. And behold, for all the other rings we also
don't enable it!

Leave the handling code itself in place just to be consistent with the
existing mess though. And in case someone feels like fixing it all up.

This has been errornously enabled in

commit 12638c57f31952127c734c26315e1348fa1334c2
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Tue May 28 19:22:31 2013 -0700

    drm/i915: Enable vebox interrupts

Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>