msm: mdss: add support to set the parent of DP pixel clock RCG

DP pixel RCG clock has multiple parents as per the clock plan
and doesn't have a fixed frequency table defined in the common
clock driver. For targets which use common clock framework like
SDM660, the parent of such RCGs need to be set by the client
itself. Add support to set the parent of DP pixel RCG clock.

Change-Id: I00ec66725ab40de28db84593c111d402a7bd7ac8
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>