msm: mdss: use u64 instead of u32 for bitclock rate calculation

Bitclock calculation for high resolution panel overflows the
32bit variable which causes invalid pixel clock and byte clock
setting. Updating it to 64bit fixes the overflow issue.

Change-Id: If1f6fa3200783b87022366580fc18bfe6e3ddd8d
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>