clk: qcom: Add support to set rcg parent src for dp_pixel clock

For set rate, dp_pixel_clk_src rcg requires the correct parent
source to configured so add support for the same.

Change-Id: I9c8ae2904b47dbe0bc6845e2ca38fbd2f126a7e5
Signed-off-by: Amit Nischal <anischal@codeaurora.org>