drm/i915/dp: DPTX writes Swing/Pre-emphs(DPCD 0x103-0x106) requested during PHY Layer testing

Source needs to write DPCD 103-106 after receiving a PHY request to change
swing/pre-emphasis after reading DPCD 206-207. This is especially needed if
there is a retimer between source and sink and the retimer implements AUX_CH
interception scheme to manage DP PHY settings (e.g. adjusting Swing/Pre-emphasis
equalization level) for DP output channel. If the source doesn't write to
DPCD 103-106, the retimer may not output the requested swing/pre-emphasis and
eventually we fail compliance.

v2: Rebase and use crtc->lane_count (Imre)

Signed-off-by: Khaled Almahallawy <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Imre Deak <[email protected]>
Signed-off-by: Imre Deak <[email protected]>

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