drm/i915/display: move crtc and dpll declarations where they belong

The definitions are in the crtc and dpll files; move the declarations to
the corresponding headers.

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/20210427120315.12342-1-jani.nikula@intel.com