drm_hwcomposer: Check for NaN refresh in VSyncWorker

... to avoid a FPE_INTDIV noticed on Cuttlefish. I believe this
happens when the VSyncWorker starts working before an active
has been set.

Signed-off-by: Jason Macnak <natsu@google.com>
2 jobs for main in 17 minutes and 24 seconds (queued for 9 seconds)