drm/etnaviv: fix power register offset on GC300

Older GC300 revisions have their power registers at an offset of 0x200
rather than 0x100. Add new gpu_read_power and gpu_write_power functions
to encapsulate accesses to the power addresses and fix the addresses.

Signed-off-by: Doug Brown <doug@schmorgal.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>