drm/amd/display: fix LFC tearing at top of screen

Tearing occurred because new VTOTAL MIN/MAX was being programmed
too early.
The flip can happen within the VUPDATE high region, and the new min/max
would take effect immediately. But this means that frame is not variable
anymore, and tearing would occur when the flip actually happens.

The fixed insert duration should be programmed on the first VUPDATE
interrupt instead.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>