msm: mdss: fix truncation of 64 bit for clk rate

Pixel clock can have a 64 bits value; this
value is truncated in some parts of the
driver where it is handled as a 32 bits value.
This change corrects the driver to make sure we
always handle the pixel clock as a 64 bits.

Change-Id: Ia97cb849ac7ce08a5c387eb11b1b01aad36244a0
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>