drm/etnaviv: rework clock initialization

The reset path wants to initialize the clock control register regardless
of the DYNAMIC_FREQUENCY_SCALING feature, so don't call clock update, but
explicitly load the register.

Also disabling of the debug registers is moved into the reset function,
so we always get to the same state after a GPU reset. This means the
clock update function should not touch the bits already set in the clock
control register, but instead only update the scaling bits.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>