drm/msm: higher values of pclk can exceed 32 bits when multiplied by a factor

Make the pclk_rate u64 to accommodate higher pixel clock
rates.

Changes in v3:
- Converted pclk_rate to u32 (Archit)
- Rebase on dsi cleanup set in msm-next

Cc: Sibi Sankar <sibis@codeaurora.org>
Cc: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>