gpu: pvr: bail out from SGXOSTimer if it's been canceled

gpu: pvr: bail out from SGXOSTimer if it's been canceled

At the moment cancelling the timer is done in a synchronous way, so
we don't need to have the extra check. But a later patch will add
additional locking around the HW interrupt from where the timer can
be cancelled as well. Because of lock dependency issues described
in that patch we have to switch to an asynchronous way of cancelling
this timer and it means we need to have the extra check.

Also reset the flag indicating that the timer is cancelled when
destroying it. Currently we are both cancelling and destroying the
timer, but we really should just require calling the destroy function.

Signed-off-by: Imre Deak <imre.deak@nokia.com>