drm_hwcomposer: clean Importer inherited classes

Move some common logic from Importer class implementations
into DrmGenericImporter class:
 - reused generic constructors and destructors for derived
   classes
 - formed common Init() logic for all derived classes
 - removed unused gralloc_ and drm_ variables from derived
   classes
 - made drm_ protected for base class to be reused in derived

Signed-off-by: Mykhailo Sopiha <mykhailo.sopiha@linaro.org>
diff --git a/platformhisi.h b/platformhisi.h
index 14a58b9..9dfea89 100644
--- a/platformhisi.h
+++ b/platformhisi.h
@@ -29,23 +29,15 @@
 
 class HisiImporter : public DrmGenericImporter {
  public:
-  HisiImporter(DrmDevice *drm);
-  ~HisiImporter() override;
-
-  int Init();
+  using DrmGenericImporter::DrmGenericImporter;
 
   int ImportBuffer(buffer_handle_t handle, hwc_drm_bo_t *bo) override;
-
   bool CanImportBuffer(buffer_handle_t handle) override;
 
  private:
   uint64_t ConvertGrallocFormatToDrmModifiers(uint64_t flags, bool is_rgb);
 
   bool IsDrmFormatRgb(uint32_t drm_format);
-
-  DrmDevice *drm_;
-
-  const gralloc_module_t *gralloc_;
 };
 }  // namespace android