platforms/drm: Refactor event dispatching code

There are a couple of reasons not to use the lambda:

* It is unnecessary. The DrmGpu has the DRM file descriptor
* If a crash occurs somewhere in the lambda, the backtrace will be hard
  to read
* Instead of processing events in the destructor of the DrmBackend
  class, we should keep dispatching events without involving
  QCoreApplication::processEvents() until all page flips are completed.