drm_hwcomposer: Move CalcPixOps into Backend

The CalcPixOps is used only for display validation which is implemented
in the generic backend.

Signed-off-by: Matvii Zorin <matvii.zorin@globallogic.com>
diff --git a/backend/Backend.h b/backend/Backend.h
index 2725809..8658efa 100644
--- a/backend/Backend.h
+++ b/backend/Backend.h
@@ -32,6 +32,10 @@
       const std::map<uint32_t, DrmHwcTwo::HwcLayer *> &z_map);
   virtual bool IsClientLayer(DrmHwcTwo::HwcDisplay *display,
                              DrmHwcTwo::HwcLayer *layer);
+
+ protected:
+  uint32_t CalcPixOps(const std::map<uint32_t, DrmHwcTwo::HwcLayer *> &z_map,
+                      size_t first_z, size_t size);
 };
 }  // namespace android