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/platform.h b/platform.h
index 547a297..a58d62e 100644
--- a/platform.h
+++ b/platform.h
@@ -49,6 +49,9 @@
   // Note: This can be called from a different thread than ImportBuffer. The
   //       implementation is responsible for ensuring thread safety.
   virtual int ReleaseBuffer(hwc_drm_bo_t *bo) = 0;
+
+  // Checks if importer can import the buffer.
+  virtual bool CanImportBuffer(buffer_handle_t handle) = 0;
 };
 
 class Planner {