drm/i915/mtl: Update MBUS_DBOX credits

drm/i915/mtl: Update MBUS_DBOX credits

Display version 14 platforms have different credits values
compared to ADL-P. Update the credits based on pipe usage.

v2: Simplify DBOX BW Credit definition(MattR)
v3:
 - Simplify only pipe per dbuf bank check(MattR)
 - Skip modeset check to handle the case when a new pipe within
   dbuf bank gets added/removed.(MattR)

Bspec: 49213

Cc: Jose Roberto de Souza <jose.souza@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Original Author: Caz Yokoyama
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913183341.908028-5-radhakrishna.sripada@intel.com