drm/i915: Extract intel_crtc_dotclock()

Extract intel_crtc_dotclock() from ddi_dotclock_get(). We'll reuse
this during state computation in order to determine the actual final
dotclcok after the DPLL computation has been done (which may not give
us the exact same port_clock that we fed in).

v2: Add the prototype

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504123350.13235-1-ville.syrjala@linux.intel.com


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