drm/etnaviv: enable GPU module level clock gating support

Enable GPU module level hardware clock gating, using the conditions
found in the galcore v5 driver.

v2 lst: Split out clock gating enable into separate function, as
there might be more conditions needed for new hardware.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>