drm/i915: Convert bandwidth state to global state

Now that we have the more formal global state thing let's
use if for memory bandwidth tracking. No real difference
to the current private object usage since we already
tried to avoid taking the single serializing lock needlessly.
But since we're going to roll the global state out to more
things probably a good idea to unify the approaches a bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120174728.21095-16-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>