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/compositor/DrmDisplayCompositor.cpp b/compositor/DrmDisplayCompositor.cpp
index 586ebe0..4e7fe0d 100644
--- a/compositor/DrmDisplayCompositor.cpp
+++ b/compositor/DrmDisplayCompositor.cpp
@@ -286,11 +286,11 @@
         break;
       }
       DrmHwcLayer &layer = layers[source_layers.front()];
-      if (!layer.buffer) {
+      if (!layer.FbIdHandle) {
         ALOGE("Expected a valid framebuffer for pset");
         break;
       }
-      fb_id = layer.buffer.FbIdHandle->GetFbId();
+      fb_id = layer.FbIdHandle->GetFbId();
       fence_fd = layer.acquire_fence.get();
       display_frame = layer.display_frame;
       source_crop = layer.source_crop;