drm/i915: Relocate intel_crtc_dotclock()

intel_crtc_dotclock() is a bit misplaced. In lieu of a better
place let's just move it next to its friends in intel_display.c.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-2-ville.syrjala@linux.intel.com