drm_hwcomposer: Rename DrmProperty::immutable()

To be a little more precise, add an 'is_' prefix

Change-Id: Idd8fe45a4dfba1cd778b4ed6b761ec489697c31a
Signed-off-by: Sean Paul <seanpaul@chromium.org>
diff --git a/drmdisplaycompositor.cpp b/drmdisplaycompositor.cpp
index a1ccdc7..8599363 100644
--- a/drmdisplaycompositor.cpp
+++ b/drmdisplaycompositor.cpp
@@ -359,7 +359,8 @@
         }
       }
 
-      if (plane->zpos_property().id() && !plane->zpos_property().immutable()) {
+      if (plane->zpos_property().id() &&
+          !plane->zpos_property().is_immutable()) {
         ret = drmModeAtomicAddProperty(pset, plane->id(),
                                        plane->zpos_property().id(),
                                        source_layers.front()) < 0;