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

[Why]
Underflow observed when using a display with a large vblank region
and low refresh rate

[How]
Simplify calculation of vblank_nom

Increase value for VBlankNomDefaultUS to 800us

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Daniel Miess <daniel.miess@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>