drm_hwcomposer: cleanup hwcutils

Remove DrmHwcBuffer class. Wrap remaining logic into
DrmHwcLayer class.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/drm/DrmPlane.cpp b/drm/DrmPlane.cpp
index 5443e7e..6433fb6 100644
--- a/drm/DrmPlane.cpp
+++ b/drm/DrmPlane.cpp
@@ -231,7 +231,7 @@
     }
   }
 
-  uint32_t format = layer->buffer->format;
+  uint32_t format = layer->buffer_info.format;
   if (!IsFormatSupported(format)) {
     ALOGV("Plane %d does not supports %c%c%c%c format", id_, format,
           format >> 8, format >> 16, format >> 24);