drm/i915/bw: abstract intel_bw_qgv_point_peakbw()

Add intel_bw_qgv_point_peakbw() helper to avoid looking at struct
intel_bw_state internals outside of intel_bw.c.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/49a723e0f23e06a6045f8f9e0d06648a6bc899c7.1750847509.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>