drm_hwcomposer: clang-tidy: enable cppcoreguidelines-* checks

Our code isn't ready for the following checks therefore keep disabled
-cppcoreguidelines-pro-bounds-array-to-pointer-decay
-cppcoreguidelines-pro-bounds-constant-array-index
-cppcoreguidelines-pro-bounds-pointer-arithmetic
-cppcoreguidelines-pro-type-cstyle-cast
-cppcoreguidelines-pro-type-vararg
-cppcoreguidelines-avoid-magic-numbers
-cppcoreguidelines-macro-usage
-cppcoreguidelines-avoid-c-arrays

+ fixed existing tidy warnings for these checks.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/drm/ResourceManager.cpp b/drm/ResourceManager.cpp
index 3e16fbb..50c33aa 100644
--- a/drm/ResourceManager.cpp
+++ b/drm/ResourceManager.cpp
@@ -46,7 +46,7 @@
       std::ostringstream path;
       path << path_pattern << idx;
 
-      struct stat buf;
+      struct stat buf {};
       if (stat(path.str().c_str(), &buf))
         break;
 
@@ -75,8 +75,8 @@
 
 int ResourceManager::AddDrmDevice(std::string const &path) {
   std::unique_ptr<DrmDevice> drm = std::make_unique<DrmDevice>();
-  int displays_added;
-  int ret;
+  int displays_added = 0;
+  int ret = 0;
   std::tie(ret, displays_added) = drm->Init(path.c_str(), num_displays_);
   if (ret)
     return ret;