hexagon: ssm-conv fix for large prompts (llama/23307)

* hexagon: remove gathers and better handling of vtcm in ssm-conv

* hexagon: relax ssm-conv gating requirements

* hexagon: add new prefill ssm-conv backend test

* hexagon: remove trailing white space

* hex-rope: uninline rope_cache_init, otherwise it breaks after rebaseing with SSM_CONV changes

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Co-authored-by: Max Krasnyansky <maxk@qti.qualcomm.com>