Partially revert "drm/amd/display: Fix possible underflow for displays with large vblank"

This partially reverts commit de231189 ("drm/amd/display: Fix
possible underflow for displays with large vblank").

[Why]
The increased value of VBlankNomDefaultUS causes underflow at the
desktop of an IP KVM setup

[How]
Change the value from 800 back to 668

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Daniel Miess <daniel.miess@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>