drm/i915: hw state readout&check support for cpu_transcoder

drm/i915: hw state readout&check support for cpu_transcoder

This allows us to drop a bunch of ugly hacks and finally implement
what

commit cc464b2a17c59adedbdc02cc54341d630354edc3
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Jan 25 16:59:16 2013 -0200

    drm/i915: set TRANSCODER_EDP even earlier

tried to achieve, but that was reverted again in

commit bba2181c49f1dddf8b592804a1b53cc1a3cf408a
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Mar 22 10:53:40 2013 +0100

    Revert "drm/i915: set TRANSCODER_EDP even earlier"

Now we should always have a consistent cpu_transcoder in the
pipe_config.

v2: Fix up the code as spotted by Paulo:
- read the register for real
- assign the right pipes
- break out if the hw state doesn't make sense

v3: Shut up gcc.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>