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.cpp b/platformdrmgeneric.cpp
index 503c04a..2fcbe40 100644
--- a/platformdrmgeneric.cpp
+++ b/platformdrmgeneric.cpp
@@ -60,6 +60,10 @@
     ALOGE("Failed to open gralloc module");
     return ret;
   }
+
+  ALOGI("Using %s gralloc module: %s\n", gralloc_->common.name,
+        gralloc_->common.author);
+
   return 0;
 }