drm/i915/cdclk: abstract intel_cdclk_bw_min_cdclk()

Add intel_cdclk_bw_min_cdclk() helper to avoid looking at struct
intel_cdclk_state internals outside of intel_cdclk.c.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/d07499174ebe55fa8fb98d4cb5ff541b6f5ec95b.1750847509.git.jani.nikula@intel.com


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