drm_hwcomposer: Move DrmHwcTwo internal classes outside private section

Making backend-validation for HwcDisplay is required access to
the internal class outside DrmHwcTwo. There is no need to keep other
internal classes in the private section in this case.

Signed-off-by: Matvii Zorin <matvii.zorin@globallogic.com>
diff --git a/include/drmhwctwo.h b/include/drmhwctwo.h
index 62ee85a..5f9f6c5 100644
--- a/include/drmhwctwo.h
+++ b/include/drmhwctwo.h
@@ -40,7 +40,6 @@
 
   HWC2::Error Init();
 
- private:
   class HwcLayer {
    public:
     HWC2::Composition sf_type() const {
@@ -299,6 +298,7 @@
     DrmDevice *drm_;
   };
 
+ private:
   static DrmHwcTwo *toDrmHwcTwo(hwc2_device_t *dev) {
     return static_cast<DrmHwcTwo *>(dev);
   }