drm/i915/wm: abstract intel_dbuf_pmdemand_needs_update()

drm/i915/wm: abstract intel_dbuf_pmdemand_needs_update()

Add intel_dbuf_pmdemand_needs_update() helper to avoid looking at struct
intel_dbuf_state internals outside of skl_watermark.c.

With this, we can also move to_intel_dbuf_state(),
intel_atomic_get_old_dbuf_state(), and intel_atomic_get_new_dbuf_state()
inside skl_watermark.c.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/b493f259d0d3db047151fee18d7e801ad469fa88.1750847509.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>