platforms/drm: only use glDrawBuffer with desktop GL

That function is not available with GLES, and calling it is only necessary
for NVidia.


(cherry picked from commit 1626d7a5)

src/plugins/platforms/drm/egl_gbm_backend.cpp

@@ -359,7 +359,9 @@ bool EglGbmBackend::makeContextCurrent(const Output::RenderData &render) const
         qCCritical(KWIN_DRM) << "eglMakeCurrent failed:" << getEglErrorString();
         return false;
     }
+    if (!GLPlatform::instance()->isGLES()) {
         glDrawBuffer(GL_BACK);
+    }
     return true;
 }