drm/i915/cdclk: abstract intel_cdclk_read_hw()

Add intel_cdclk_read_hw() function to avoid looking at struct
intel_cdclk_state internals outside of intel_cdclk.c.

intel_cdclk_init_hw() would be a better name, but we already have that.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/ef720d37bfeee933d59b64e382dc976f3c9fade1.1750847509.git.jani.nikula@intel.com


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