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.cpp b/drmproperty.cpp
index 9a28374..e276f56 100644
--- a/drmproperty.cpp
+++ b/drmproperty.cpp
@@ -95,7 +95,7 @@
   }
 }
 
-bool DrmProperty::immutable() const {
+bool DrmProperty::is_immutable() const {
   return id_ && (flags_ & DRM_MODE_PROP_IMMUTABLE);
 }