drm/amd/display: make flip_timestamp_in_us a 64-bit variable

[Why]
This variable currently overflows after about 71 minutes. This doesn't
cause any known functional issues but it does make debugging more
difficult.

[How]
Make it a 64-bit variable.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Josip Pavic <josip.pavic@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>