drm/etnaviv: reduce reset delay

After reset assertion, we only have to wait for the reset signals to
propagate through the GPU before deasserting the reset again. A few
hundred clock cycles should be more than enough. Replace the msleep(1),
which can actually take about 30 ms on i.MX6Q in some configurations,
with an usleep_range of a few microseconds. If the delay was too short,
the FE would not be idle afterwards, and the reset would be retried.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>