msm: mdss: dsi: fix clk_rate to use u64 instead of u32

Commit f9c04f16 ("msm: mdss: dsi: add
support for multiple panel timing settings") changed the
mdss-dsi-panel-clockrate parameter read from u64 to u32 during the
restructuring to use panel timings struct. Revert it back to use u64.

Change-Id: I4d0a97f306e10854dfc169550532bf19772e8692
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>