drm/amd/display: Fix inconsistent timestamp type

[WHY]
An unsigned int timestamp variable is assigned with an unsigned
long long value. Also, the assignment directly converts the
tick value to us without using built-in get elapsed time function.

[HOW]
Cast the assigned value correctly and also use built-in function
to get the timestamp in the unit we want.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Angus Wang <Angus.Wang@amd.com>