drm/i915: add power monitoring support

Add power monitoring support to the i915 driver for use by the IPS
driver.  Export the available power info to the IPS driver through a few
new inter-driver hooks.  When used together, the IPS driver and this
patch can significantly increase graphics performance on Ironlake class
chips.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[anholt: Fixed 32-bit compile.  stupid obfuscating div_u64()]
Signed-off-by: Eric Anholt <eric@anholt.net>