adreno_tz: Correct acc_relative_busy calculation

Current acc_relative_busy calculation is causing integer overflow
in 32 bit system. "stats->busy_time * stats->current_frequency"
results in a value which is beyond the 32 bit range.

Typecasting the value to u64 to avoid overflow.

Change-Id: Id97da02bef608787ceb7c9751bbfc203af56deb1
Signed-off-by: Harshitha Sai Neelati <hsaine@codeaurora.org>