drm/i915: Disable FIFO underrun reporting around IBX transcoder B workaround

Doing the IBX transcoder B workaround causes underruns on
pipe/transcoder A. Just hide them by disabling underrun reporting for
pipe A around the workaround.

It might be possible to avoid the underruns by moving the workaround
to be applied only when enabling pipe A. But I was too lazy to try it
right now, and the current method has been proven to work, so didn't
want to change it too hastily.

Note that this can re-enable underrun reporting on pipe A if was
already disabled due to a previous actual underrun. But that's OK, we
may just get a second underrun report if another real underron occurrs
on pipe A.

v2: Note that pipe A underruns can get re-enabled due to this (Jani)

Signed-off-by: Ville Syrjälä <[email protected]>
Reviewed-by: Jesse Barnes <[email protected]> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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