drm/admkfd use modern ktime accessors

getrawmonotonic64() and get_monotonic_boottime64() are deprecated
because of the nonstandard naming.

The replacement functions ktime_get_raw_ns() and ktime_get_boot_ns()
also simplify the callers.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com&gt;.>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>