drm/amd/display: DML21 Update Prefetch Calculations

[Why/How]
Mismatch between mode support and mode programming occurs.
Mode support would calculate higher row vblank than mode programming.
As a result, mode programming fails and hardware isn't properly programmed.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Austin Zheng <Austin.Zheng@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>