drm/i915/dpll: move shared dpll state verification to intel_dpll_mgr.c

Keep the shared dpll implementation details together by moving the dpll
state verification to intel_dpll_mgr.c. Also declutter intel_display.c.

v2: intel_shared_dpll_verify_state -> intel_shared_dpll_state_verify (Ville)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/85b02186f1269dd374d11db35900130547a5f2c6.1655372759.git.jani.nikula@intel.com