On gen11 we only needed to program MBus credits into MBUS_ABOX_CTL
during display initialization, but on gen12 we're now supposed to
program the same values into MBUS_ABOX1_CTL and MBUS_ABOX2_CTL as well.

v2:
 - Program registers with rmw to preserve contents of unrelated bits.
 - Switch to the new display uncore helpers.

Bspec: 49213
Bspec: 50096
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204011032.582737-2-matthew.d.roper@intel.comReviewed-by: Matt Atwood <matthew.s.atwood@intel.com>