drm/i915: move *_crtc_clock_get() to intel_dpll.c

Considering what the functions do, intel_dpll.c is a more suitable
location, and lets us make some functions static while at it.

This also means intel_display.c no longer does any DPIO access.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231114104534.4180144-1-jani.nikula@intel.com