drm/amd/display: Actually do immediate vblank disable

[Why]

The `vblank_config.offdelay` field follows the same semantics as the
`drm_vblank_offdelay` parameter. Setting it to 0 will never disable
vblank.

[How]

Set `offdelay` to a positive number.

Fixes: e45b6716 ("drm/amd/display: use a more lax vblank enable policy for DCN35+")

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>