drm_hwcomposer: Add checking if we can import a buffer

Add CanImportBuffer() function to the Importer interface.
Platform specific importer should check in this function if it can
import given buffer_handle_t. For example platformhisi will return
false for buffers without GRALLOC_USAGE_HW_FB.

This function should be used on ValidateDisplay step to avoid the
need of 'fake-importing' of buffers.

Signed-off-by: Alexey Firago <alexey_firago@mentor.com>
diff --git a/platformhisi.h b/platformhisi.h
index 2b2d439..927da17 100644
--- a/platformhisi.h
+++ b/platformhisi.h
@@ -36,6 +36,8 @@
 
   int ImportBuffer(buffer_handle_t handle, hwc_drm_bo_t *bo) override;
 
+  bool CanImportBuffer(buffer_handle_t handle) override;
+
  private:
   DrmDevice *drm_;