drm/i915/cdclk: abstract intel_cdclk_force_min_cdclk()

Add intel_cdclk_force_min_cdclk() helper to avoid modifying struct
intel_cdclk_state internals outside of intel_cdclk.c.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/0bf8a94a1a7d3ac564406ba427d12c4c8eefb5bb.1750847509.git.jani.nikula@intel.com


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