drm/i915: fix GMCH power reporting

drm/i915: fix GMCH power reporting

The IPS driver needs to know the current power consumption of the GMCH
in order to make decisions about when to increase or decrease the CPU
and/or GPU power envelope.  So fix up the divisions to save the results
so the numbers are actually correct (contrary to some earlier comments
and code, these functions do not modify the first argument and use it
for the result).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org