arm64/fpsimd: Factor out {sve,sme}_state_size() helpers

arm64/fpsimd: Factor out {sve,sme}_state_size() helpers

In subsequent patches we'll need to determine the SVE/SME state size for
a given SVE VL and SME VL regardless of whether a task is currently
configured with those VLs. Split the sizing logic out of
sve_state_size() and sme_state_size() so that we don't need to open-code
this logic elsewhere.

At the same time, apply minor cleanups:

* Move sve_state_size() into fpsimd.h, matching the placement of
  sme_state_size().

* Remove the feature checks from sve_state_size(). We only call
  sve_state_size() when at least one of SVE and SME are supported, and
  when either of the two is not supported, the task's corresponding
  SVE/SME vector length will be zero.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-8-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>