drm/amd/display: Implement dm_get_timestamp

We use this to ensure we wait at least 500ms in between eDP
disable/enable.
Signed-off-by: NHarry Wentland <harry.wentland@amd.com>
Reviewed-by: NTony Cheng <Tony.Cheng@amd.com>
Acked-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_services.c

@@ -37,8 +37,10 @@
 
 unsigned long long dm_get_timestamp(struct dc_context *ctx)
 {
-	/* TODO: return actual timestamp */
-	return 0;
+	struct timespec64 time;
+
+	getrawmonotonic64(&time);
+	return timespec64_to_ns(&time);
 }
 
 void dm_perf_trace_timestamp(const char *func_name, unsigned int line)