drm/i915: Add pre/post plane updates for SAGV

Lets have a unified way to handle SAGV changes,
espoecially considering the upcoming Gen12 changes.

Current "standard" way of doing this in commit_tail
is pre/post plane updates, when everything which
has to be forbidden and not supported in new config
has to be restricted before update and relaxed after
plane update.

v2: - Removed unneeded returns(Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415143911.10244-5-stanislav.lisovskiy@intel.com