drm/etnaviv: fix power register offset on GC300

[ Upstream commit 61a6920bb604df3a0e389a2a9479e1e233e4461d ]

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>
Stable-dep-of: 37dc4737447a ("drm/etnaviv: hold GPU lock across perfmon sampling")
Signed-off-by: Sasha Levin <sashal@kernel.org>