drm/amd/display: Emulate Display Hotplug Hang

[WHY]
Driver reports 0 display when the virtual display is still present, and
causes P-state hang in FW.

[HOW]
When enumerating through streams, check for active planes and use that
to indicate number of displays.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Daniel Sa <Daniel.Sa@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>