drm/amdgpu: fix vblank_time when displays are off

If the displays are off, set the vblank time to max to make
sure mclk switching is enabled.  Avoid mclk getting set
to high when no displays are attached.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=101528
fixes: 09be4a52 (drm/amd/powerplay/smu7: add vblank check for mclk switching (v2))
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/amdgpu/amdgpu_cgs.c

@@ -838,6 +838,9 @@ static int amdgpu_cgs_get_active_displays_info(struct cgs_device *cgs_device,
 		return -EINVAL;
 
 	mode_info = info->mode_info;
+	if (mode_info)
+		/* if the displays are off, vblank time is max */
+		mode_info->vblank_time_us = 0xffffffff;
 
 	if (adev->mode_info.num_crtc && adev->mode_info.mode_config_initialized) {
 		list_for_each_entry(crtc,