clk: qcom: mdss: calculate pixel clock for HDMI during handoff

Calculate the current pixel clock value for the resolution
programmed in the bootloader. This value is needed for HDMI
configuration during handoff use cases.

Change-Id: I9ea4a8130b4d474c333254611732b63a0366fda3
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>