drm/bridge: ti-sn65dsi86: Don't use MIPI variables for DP link

The ti-sn65dsi86 is a bridge from MIPI to DP and thus has two links:
the MIPI link and the DP link.  The two links do not need to have the
same format or number of lanes.  Stop using MIPI variables when
talking about the DP link.

This has zero functional change because:
* currently we are hardcoding the MIPI link as unpacked RGB888 which
  requires 24 bits and currently we are not changing the DP link rate
  from the bridge's default of 8 bits per pixel.
* currently we are hardcoding both the MIPI and DP as being 4 lanes.

This is all in prep for fixing some of the above.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191218143416.v3.3.Ia6e05f4961adb0d4a0d32ba769dd7781ee8db431@changeid