drm/msm: dsi: Handle dual-channel for 6G as well

This fixes up a collision between introducing dual-channel support and
the dsi refactors. This patch applies the same dual-channel
considerations and pclk calculations to both v2 and 6G, with a bit of
abstracting for good measure.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>