drm_hwcomposer: Set return type to std::optional for BufferInfoGetters

This is a bit of code modernization. Further changes will require indication
that buffer_info is valid, and using std::optional is the most correct
approach to do that.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/drm/DrmPlane.cpp b/drm/DrmPlane.cpp
index 28f48f3..12ac44f 100644
--- a/drm/DrmPlane.cpp
+++ b/drm/DrmPlane.cpp
@@ -176,7 +176,7 @@
     return false;
   }
 
-  uint32_t format = layer->buffer_info.format;
+  uint32_t format = layer->buffer_info->format;
   if (!IsFormatSupported(format)) {
     ALOGV("Plane %d does not supports %c%c%c%c format", GetId(), format,
           format >> 8, format >> 16, format >> 24);