drm/i915: Cleanup if the EDP transcoder has a bobug input value

In the case where the hardware has been wrongly programmed and the EDP
TRANS_DDI_FUNC_CTL register has a bogus value in its EDP Input field, we
were using the pipe variable uninitialized.

In this case, shutdown the transcoder. It will be programmed correctly
the next time we try to enabled eDP.

Note from Paulo's review: Wrong modeset sequence can easily lead to
frozen machines hence the disable_ddi call might be risky. But since
things are awry already, doesn't matter too much.

Signed-off-by: Damien Lespiau <[email protected]>
Reviewed-by: Paulo Zanoni <[email protected]>
[danvet: Add note about Paulo's caution about potential hangs.]
Signed-off-by: Daniel Vetter <[email protected]>

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