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/platformdrmgeneric.h b/platformdrmgeneric.h
index 233ba55..c6d2be6 100644
--- a/platformdrmgeneric.h
+++ b/platformdrmgeneric.h
@@ -38,9 +38,10 @@
   uint32_t ConvertHalFormatToDrm(uint32_t hal_format);
   uint32_t DrmFormatToBitsPerPixel(uint32_t drm_format);
 
- private:
+ protected:
   DrmDevice *drm_;
 
+ private:
   const gralloc_module_t *gralloc_;
 };
 }  // namespace android