drm/msm/sde: list hardwired mixer connections

Express hardwired connections in the hardware catalog between
layer mixers, dspps, and pingpong blocks. Also list out which
mixer pairings are valid.

Change-Id: I06b9c57d79b678df1372293247f2be7ab34950ca
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>