disp: pll: remove unsupported dividers for DSI pixel clock

Remove dividers that are not recommended for DSI DPHY mode
when setting up the clock tree for the DSI pixel clock.

Change-Id: I2563a35ece541c1f5b46c72af7bd2cc79e72a90e
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>