drm/i915: drop intel_display.h include from intel_dpll_mgr.h

Use forward declarations instead.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116171434.20516-2-jani.nikula@intel.com