drm/amd/display: Add get_current_time interface to dmub_srv

[Why]
Need to get current DMUB time.

[How]
Add get_current_time interface to dmub_srv.

v2: drop whitespace changes (Alex)

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>