video: fsl-dcu-fb: remove underrun interrupt handlingtoradex_vf_4.0

The current buffer underrun interrupt handling seems bogus in
multiple ways: Disabling and reenabling the controller seems a
questionable procedure and is also not documented. Furthermore,
to apply a DCU mode, a register transfer would be necessary
(UPDATE_MODE).

The current implementation probably even introduce races: If
a underrun happens just when enabling the controller (which has
been observed to happen quite often during initialization), the
register could still be in transfer mode. A write to a register
could still be transferred in this situation, which might lead
that the DCU off mode is transferred. Since the interrupt handler
does not initiate another register transfer, the controller would
stay off in this situation...

Rare conditions have been observed in which the controller ends
up in test mode. The race condition outlined above describes a
sequence in which the controller ends up beeing disabled. However,
before cd586e4cf9dc ("video: fsl-dcu-fb: fix operating mode off)
the DCU state off and test have been mixed up, hence this patch
could in fact fix that issue.

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