GPU: allow VL-SDPA fallback without DPAS

[Chuansheng-Liu]

Summary

• remove the supports_immad gate so SDPA→VL-SDPA runs even on non-DPAS GPUs
• pass CMFLA_USE_STUB_KERNEL to the CM kernel so it returns early when DPAS isn’t available
• drop DPAS-only static_asserts and update vlsdpa unit tests/docs accordingly

Testing

• ./bin/intel64/Release/ov_gpu_unit_tests --gtest_filter="smoke_vlsdpa_gpu_test/vlsdpa_gpu_test.*" (currently fails: CM stub still hits DPAS path;)