drm/amdkfd: Convert timestamping to use 64bit time accessors

Convert the timestamping in the amdkfd driver to use a timespec64 and 64bit
time accessors.

Although the existing code is completely safe beyond y2038 because it deals
with monotonic time, this patch is still needed in order to kill off all uses
of struct timespec.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>