i915: reprogram power monitoring registers on resume

Fixes issue where i915_gfx_val was reporting values several
orders of magnitude higher than physically possible (without
leaving scorch marks on my thighs at least.)

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>