drm/i915: Implement display w/a #1143

Apparently SKL/KBL/CFL need some manual help to get the
programmed HDMI vswing to stick. Implement the relevant
workaround (display w/a #1143).

Note that the relevant chicken bits live in a transcoder register
even though the bits affect a specific DDI port rather than a
specific transcoder. Hence we must pick the correct transcoder
register instance based on the port rather than based on the
cpu_transcoder.

Also note that for completeness I included support for DDI A/E
in the code even though we never have HDMI on those ports.

v2: CFL needs the w/a as well (Rodrigo and Art)

Cc: Rodrigo Vivi <[email protected]>
Cc: Art Runyan <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Rodrigo Vivi <[email protected]>

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