output/kms: Don't attemp to add common modes on connectors without modes

We have no way to sanely add safe modes if there are no modes we can compare with, thus don't try.

Fixes the following crash:

 #0 are_all_modes_equally_sized at ../src/backends/native/meta-output-kms.c:284
 #1 maybe_add_fallback_modes at ../src/backends/native/meta-output-kms.c:310
 #2 init_output_modes at ../src/backends/native/meta-output-kms.c:347
 #3 meta_output_kms_new at ../src/backends/native/meta-output-kms.c:414
 #4 init_outputs at ../src/backends/native/meta-gpu-kms.c:332
 #5 meta_gpu_kms_read_current at ../src/backends/native/meta-gpu-kms.c:368
 #6 meta_gpu_kms_new at ../src/backends/native/meta-gpu-kms.c:403
 #7 create_gpu_from_udev_device at ../src/backends/native/meta-backend-native.c:461
 #8 init_gpus at ../src/backends/native/meta-backend-native.c:551
 #9 meta_backend_native_initable_init at ../src/backends/native/meta-backend-native.c:632

Fixes: 877cc3eb Related: https://bugzilla.redhat.com/show_bug.cgi?id=2127801

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Cc: @superm1