i915: Silence unused variable warning in non-debug builds.

Fixes this GCC warning.
i915_vtbl.c: In function 'i915_assert_not_dirty':
i915_vtbl.c:670: warning: unused variable 'dirty'

src/mesa/drivers/dri/i915/i915_vtbl.c

@@ -669,6 +669,7 @@ i915_assert_not_dirty( struct intel_context *intel )
    struct i915_context *i915 = i915_context(&intel->ctx);
    GLuint dirty = get_dirty(&i915->state);
    assert(!dirty);
+   (void) dirty;
 }
 
 void