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/drmproperty.h b/drmproperty.h
index 36cd63d..5b40c4c 100644
--- a/drmproperty.h
+++ b/drmproperty.h
@@ -46,7 +46,7 @@
   std::string name() const;
 
   std::tuple<int, uint64_t> value() const;
-  bool immutable() const;
+  bool is_immutable() const;
 
  private:
   class DrmPropertyEnum {