drm/i915: Move the dbuf pre/post plane update

drm/i915: Move the dbuf pre/post plane update

Encapsulate the dbuf state more by moving the pre/post
plane functions out from intel_display.c. We stick them
into intel_pm.c since that's where the rest of the code
lives for now.

Eventually we should add a new file for this stuff at which
point we also need to decide if it makes sense to even split
the wm code from the ddb code, or to keep them together.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225171125.28885-11-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>