Unfortunately the public Bspec misses totally out on the same language
for HDMIB. Internal Bspec also mentions that one of the bad
side-effects is that DPx can fail to light up on transcoder A if HDMIx
is disabled but using transcoder B.

I've found this while reviewing Bsepc. We already implement the same
workaround for the DP ports.

Also replace a magic 1 with PIPE_B I've found while looking through the
code.

v2: Implement suggestions from Chris Wilson:
- add pipe variable to cut down on code noise
- write the reg value twice to w/a hw issues (Bspec is unclear on
  which bit actually require the write twice stuff, but better be
  paranoid about it)
- untangle the if logic

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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