drm/amd/display: Fix nanosec stat overflow

[ Upstream commit 14d68acf

 ]

[Why]
Nanosec stats can overflow on long running systems potentially causing
statistic logging issues.

[How]
Use 64bit types for nanosec stats to ensure no overflow.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>