drm/i915/wm: move wm state verification to intel_pm.c

By moving wm state verification to intel_pm.c, we can make a bunch of
functions static, hiding the wm details better. Also declutter
intel_display.c.

v2: intel_wm_state_verify -> intel_wm_verify_state (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/2a7e3141e87181c07eaddcd9c352b8810550b0ce.1655372759.git.jani.nikula@intel.com