drm/msm: a6xx: Fix improper u64 division

This patch uses the proper do_div() macro to perform u64 division and
guards against overflow if the result is too large for the unsigned long
return type

Fixes: a2c3c0a5

 drm/msm/a6xx: Add devfreq support for a6xx
Cc: Sharat Masetty <smasetty@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>