backends/drm: fix reallocation check
When no modifiers are used at all, we shouldn't check them. This bug causes KWin to allocate lots of new memory on every new frame, which can be quite expensive (depends on driver and hardware).
On my system, once I disabled explicit modifiers (which are disabled by default), CPU usage was at 4-5% due to unnecessary re-allocations. With this patch it's down to 1-2% again (with Firefox open, which causes lots of presentation to happen).