drm/i915: CPT+ pch transcoder workaround

We need to set the timing override chicken bit after fdi link training
has completed and before we enable the transcoder. We also have to
clear that bit again after disabling the pch transcoder.

See "Graphics BSpec: vol4g North Display Engine Registers [IVB],
Display Mode Set Sequence" and "Graphics BSpec: vol4h South Display
Engine Registers [CPT, PPT], South Display Engine Transcoder and FDI
Control, Transcoder Debug and DFT, TRANS_CHICKEN_2" bit 31:

"Workaround : Enable the override prior to enabling the transcoder.
Disable the override after disabling the transcoder."

While at it, use the _PIPE macro for the other TRANS_DP register.

v2: Keep the w/a as-is, but kill the original (but wrongly placed)
workaround introduced in

commit 3bcf603f
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Wed Jul 27 11:51:40 2011 -0700

    drm/i915: apply timing generator bug workaround on CPT and PPT

and

commit d4270e57
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Tue Oct 11 10:43:02 2011 -0700

    drm/i915: export a CPT mode set verification function

Note that this old code has unconditionally set the w/a, which might
explain why fdi link training sometimes silently fails, and especially
why the auto-train did not seem to work properly.

v3: Paulo Zanoni pointed out that this workaround is also required on
the LPT PCH. And Arthur Ranyan confirmed that this workaround is
requierd for all ports on the pch, not just DP: The important part
is that the bit is set whenever the pch transcoder is enabled, and
that it is _not_ set while the fdi link is trained. It is also
important that the pch transcoder is fully disabled, i.e. we have to
wait for bit 30 to clear before clearing the w/a bit.

Hence move to workaround into enable/disable_transcoder, where the pch
transcoder gets enabled/disabled.

v4: Whitespace changes dropped.

v5: Don't run the w/a on IBX, we only need it on CPT/PPT and LPT.

v6:
- resolve conflicts with Paulo's big hsw vga rework
- s/!IBX/CPT since hsw paths are now all separate, and Paulo's patch
  to implement the equivalent w/a for LPT is already merged.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Paulo Zanoni <przanoni@gmail.com>
Cc: Arthur Ranyan <arthur.j.runyan@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v5)
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org> (v5)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>