drm/amd/pm/si: Disregard vblank time when no displays are connected

drm/amd/pm/si: Disregard vblank time when no displays are connected

When no displays are connected, there is no vblank
happening so the power management code shouldn't
worry about it.

This fixes a regression that caused the memory clock
to be stuck at maximum when there were no displays
connected to a SI GPU.

Fixes: 9003a0746864 ("drm/amd/pm: Treat zero vblank time as too short in si_dpm (v3)")
Fixes: 9d73b107a61b ("drm/amd/pm: Use pm_display_cfg in legacy DPM (v2)")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Jeremy Klarenbeek <jeremy.klarenbeek99@gmail.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>