drm/i915: fix fdi related fifo underruns on hsw

drm/i915: fix fdi related fifo underruns on hsw

Similar to

commit 37ca8d4ccd9860df0747aa2ea281a3c9c4bf8826
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Fri Oct 30 19:20:27 2015 +0200

    drm/i915: Enable PCH FIFO underruns later on ILK/SNB/IVB

we can only enable fifo underrun reporting when using the fdi/lpt
after everything is set up and after a bit of waiting. The waiting
is required, enabling it right after enabling encoders will first trigger
an underrun on the pch and then, 1 frame later, an underrun on the
cpu. Two vblank waits after encoder enabling seems enough to curb it.

And similar to

Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Fri Nov 20 22:09:18 2015 +0200

    drm/i915: Suppress spurious CPU FIFO underruns on ILK-IVB

we also need to make sure cpu fifo underrun reporting is disabled when
enabling the fdi rx/tx and pch transcoder&port. But somehow this is
only needed when enabling, not also when disabling.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448705139-12534-1-git-send-email-daniel.vetter@ffwll.ch
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91578
Tested-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>