drm/i915/mpllb: move mpllb state check to intel_snps_phy.c

Keep the mpllb implementation details together in intel_snps_phy.c. Also
declutter intel_display.c.

v2: intel_mpllb_verify_state -> void intel_mpllb_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/e7340bb0e399aeb2676c4820461187eeb1d4db15.1655372759.git.jani.nikula@intel.com