drm_hwcomposer: avoid using signed errno on uint32 type

DrmGenericImporter::ConvertHalFormatToDrm() should not return negative values.

 - Use DRM_FORMAT_INVALID instead of -EINVAL
 - Check DrmGenericImporter::ConvertHalFormatToDrm() result value in
   DrmGenericImporter::ImportBuffer()

Signed-off-by: Roman Stratiienko <roman.stratiienko@globallogic.com>
diff --git a/platform/platformmeson.cpp b/platform/platformmeson.cpp
index 7bde5cd..10c2745 100644
--- a/platform/platformmeson.cpp
+++ b/platform/platformmeson.cpp
@@ -98,9 +98,9 @@
     return ret;
   }
 
-  int32_t fmt = ConvertHalFormatToDrm(hnd->req_format);
-  if (fmt < 0)
-    return fmt;
+  uint32_t fmt = ConvertHalFormatToDrm(hnd->req_format);
+  if (fmt == DRM_FORMAT_INVALID)
+    return -EINVAL;
 
   modifiers[0] = ConvertGrallocFormatToDrmModifiers(hnd->internal_format);