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/platformminigbm.h b/platformminigbm.h
index 25f8404..ff69f14 100644
--- a/platformminigbm.h
+++ b/platformminigbm.h
@@ -27,17 +27,8 @@
 
 class DrmMinigbmImporter : public DrmGenericImporter {
  public:
-  DrmMinigbmImporter(DrmDevice *drm);
-  ~DrmMinigbmImporter() override;
-
-  int Init();
-
+  using DrmGenericImporter::DrmGenericImporter;
   int ImportBuffer(buffer_handle_t handle, hwc_drm_bo_t *bo) override;
-
- private:
-  DrmDevice *drm_;
-
-  const gralloc_module_t *gralloc_;
 };
 
 }  // namespace android