drm/i915: set IDICOS to medium uncore resources

I'm seeing about a 5% FPS improvement across various benchmarks on my
IVB i3. Rumor has it that the higher end parts show even more benefit.

This derives from a patch originally given to me by Bernard. The docs
are  confusing about the definition names (ie. medium really seems like
max), but it would seem it gives more cache to the GT at the expense of
uncore. This configuration makes the split most in favor of the GT. I've
not tried the other IDICOS values.

Cc: "Kilarski, Bernard R" <bernard.r.kilarski@intel.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>