backends/drm: Fix 100% cpu usage after tty switch

The drm fd can get stuck in readable state, in which case the
QSocketNotifier will fire the activated signal as often as it can
leading to high cpu usage.

We need to read() the drm fd in order to make the socket notifier stop
firing QSocketNotifier::activated.

This change removes the m_platform->isActive() check to ensure that
drmHandleEvent() gets called, in general, it should be safe as we only
notify the outputs about completed pageflips.

BUG: 452726


(cherry picked from commit 1d8c9c62)