mdss: clock: add edp dynamic clock rate support

Add edp both main link clock and pixel clock related
support functions so that clock rate of both clock can
be configured dynamically.

Change-Id: I749c79923d7e7db1d722dab6628d58f88d178de9
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>