amdgpu: display: use modern ktime accessors

getrawmonotonic64() is deprecated because of the nonstandard naming.

The replacement functions ktime_get_raw_ns() also simplifies the callers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>