drm/amd/display: Skip backend validation for virtual monitors

[Why&How]
Virtual monitors are now being validated during set_mode.
Virtual monitors should not undergo backend validation,
as the backend is intended only for physical monitors.
Virtual sinks have no real backend part information and
should be excluded from this validation.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Chiawen Huang <chiawen.huang@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>