drm/etnaviv: always start/stop scheduler in timeout processing

The drm scheduler currently expects that the stop/start sequence is always
executed in the timeout handling, as the job at the head of the hardware
execution list is always removed from the ring mirror before the driver
function is called and only inserted back into the list when starting the
scheduler.

This adds some unnecessary overhead if the timeout handler determines
that the GPU is still executing jobs normally and just wished to extend
the timeout, but a better solution requires a major rearchitecture of the
scheduler, which is not applicable as a fix.

Fixes: 135517d3 ("drm/scheduler: Avoid accessing freed bad job.")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Russell King <rmk+kernel@armlinux.org.uk>