drm/amdgpu: fix vblank_time when displays are off

drm/amdgpu: fix vblank_time when displays are off

commit beb3777682d5c296cc15a2a424f5a7a98476def0 upstream.

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: 09be4a5219 (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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>