drm_hwcomposer: Add z order support

Currently, the planner just pops the first available drm plane and if
that can't be used for the DrmHwcLayer it just returns error.

This proposes a slighlty smarter way to do that by trying to see if
any of the DrmPlane can be used for the DrmHwcLayer in question.

More, if the drm_plane doesn't have a fix zorder then we could re-add
him to the list of unused planes, so it could be used for hosting
other less demanding DrmHwcLayers.

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
diff --git a/drmproperty.h b/drmproperty.h
index 5e358be..f1328fe 100644
--- a/drmproperty.h
+++ b/drmproperty.h
@@ -46,6 +46,7 @@
   std::string name() const;
 
   int value(uint64_t *value) const;
+  bool immutable() const;
 
  private:
   class DrmPropertyEnum {