drm/i915: Don't deref pipe->cpu_transcoder in the hangcheck code

drm/i915: Don't deref pipe->cpu_transcoder in the hangcheck code

If we get an error event really early in the driver setup sequence,
which gen3 is especially prone to with various display GTT faults we
Oops. So try to avoid this.

Additionally with Haswell the transcoders are a separate bank of
registers from the pipes (4 transcoders, 3 pipes). In event of an
error, we want to be sure we have a complete and accurate picture of
the machine state, so record all the transcoders in addition to all
the active pipes.

This regression has been introduced in

commit 702e7a56af3780d8b3a717f698209bef44187bb0
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Tue Oct 23 18:29:59 2012 -0200

    drm/i915: convert PIPECONF to use transcoder instead of pipe

Based on the patch "drm/i915: Dump all transcoder registers on error"
from Chris Wilson:

v2: Rebase so that we don't try to be clever and try to figure out the
cpu transcoder from hw state. That exercise should be done when we
analyze the error state offline.

The actual bugfix is to not call intel_pipe_to_cpu_transcoder in the
error state capture code in case the pipes aren't fully set up yet.

v3: Simplifiy the err->num_transcoders computation a bit. While at it
make the error capture stuff save on systems without a display block.

v4: Fix fail, spotted by Jani.

v5: Completely new commit message, cc: stable.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=60021
Cc: stable@vger.kernel.org
Tested-by: Dustin King <daking@rescomp.stanford.edu>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>