[intel] Add a driconf option to cache freed buffer objects for reuse.

This is defaulted off as it has potentially large memory costs for a modest
performance gain.  Ideally we will improve DRM performance to the point where
this optimization is not worth the memory cost in any case, or find some
middle ground in caching only limited numbers of certain buffers.  For now,
this provides a modest 4% improvement in openarena on GM965 and 10% in openarena
on GM945.

src/mesa/drivers/dri/intel/intel_screen.c

@@ -56,6 +56,15 @@ PUBLIC const char __driConfigOptions[] =
    DRI_CONF_SECTION_PERFORMANCE
       DRI_CONF_FTHROTTLE_MODE(DRI_CONF_FTHROTTLE_IRQS)
       DRI_CONF_VBLANK_MODE(DRI_CONF_VBLANK_DEF_INTERVAL_0)
+      /* Options correspond to DRI_CONF_BO_REUSE_DISABLED,
+       * DRI_CONF_BO_REUSE_ALL
+       */
+      DRI_CONF_OPT_BEGIN_V(bo_reuse, enum, 0, "0:1")
+	 DRI_CONF_DESC_BEGIN(en, "Buffer object reuse")
+	    DRI_CONF_ENUM(0, "Disable buffer object reuse")
+	    DRI_CONF_ENUM(1, "Enable reuse of all sizes of buffer objects")
+	 DRI_CONF_DESC_END
+      DRI_CONF_OPT_END
    DRI_CONF_SECTION_END
    DRI_CONF_SECTION_QUALITY
       DRI_CONF_FORCE_S3TC_ENABLE(false)
@@ -66,7 +75,7 @@ PUBLIC const char __driConfigOptions[] =
    DRI_CONF_SECTION_END
 DRI_CONF_END;
 
-const GLuint __driNConfigOptions = 5;
+const GLuint __driNConfigOptions = 6;
 
 #ifdef USE_NEW_INTERFACE
 static PFNGLXCREATECONTEXTMODES create_context_modes = NULL;