Always unlock graphics buffer after binding texture during composeAndFlush

The code path for re-generating the texture already has the unlock, but the
non-regenerating code-path was missing an unlock, causing asserts when the
graphics buffer was then attempted to be locked (again) at a later point.

Change-Id: Ic65add838360079829b7e81d7c1cfe2274959059
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/painting/qplatformbackingstore.cpp

@@ -411,6 +411,7 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
         } else if (!region.isEmpty()){
             funcs->glBindTexture(GL_TEXTURE_2D, d_ptr->textureId);
             QPlatformGraphicsBufferHelper::lockAndBindToTexture(graphicsBuffer, &d_ptr->needsSwizzle, &d_ptr->premultiplied);
+            graphicsBuffer->unlock();
         }
 
         if (graphicsBuffer->origin() == QPlatformGraphicsBuffer::OriginBottomLeft)