drm_hwcomposer: clean Importer inherited classes
Move some common logic from Importer class implementations into DrmGenericImporter class:
- reused generic constructors and restructors for derived classes
- formed common Init() logic for all derived classes
- removed unused gralloc_ and drm_ variables from dericed classes
- made drm_ protected for base class to be reused in derived
Signed-off-by: Mykhailo Sopiha mykhailo.sopiha@linaro.org