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: e45b6716de ("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>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -8747,6 +8747,8 @@ static void manage_dm_interrupts(struct amdgpu_device *adev,
 
 			config.offdelay_ms = offdelay ?: 30;
 		} else {
+			/* offdelay_ms = 0 will never disable vblank */
+			config.offdelay_ms = 1;
 			config.disable_immediate = true;
 		}