drm/i915: Try to shut up more ILK underruns

drm/i915: Try to shut up more ILK underruns

Take a bigger hammer to the underrun suppression on ILK. Instead of
trying to suppress them at specific points in the modeset sequence just
silence them across the entire sequence. This gets rid of some underruns
at least on my ILK. Note that this changes SNB and IVB to follow the
same approach just to keep the code less convoluted. The difference is
that on those platforms we won't suppress CPU underruns for port A since
it doesn't seem to be necessary.

My ILK has port A eDP and two PCH HDMI ports, so I can't be sure this is
as effective on other PCH port types. Perhaps we still need some of
Daniel's extra vblank waits [2]?

I've still been able to trigger an underrun on the other pipe, but
fixing that perhaps needs the LP1+ disable trick I implemented here [1]
which never got merged.

A few details which hamper stress testing on my ILK are that sometimes
the PCH transcoder gets messed up and refuses to shut down, and sometimes
even the panel power sequencer apparently gets stuck on the always on
position.

[1] https://lists.freedesktop.org/archives/intel-gfx/2014-March/041317.html
[2] https://lists.freedesktop.org/archives/intel-gfx/2016-January/086397.html

v2: Add a note that we also get underruns when enabling PCH ports

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1459536799-18109-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>