drm/i915/display/adl_p: Correctly program MBUS DBOX A credits

Alderlake-P have different values for MBUS DBOX A credits depending
if MBUS join is enabled or not.

BSpec: 50343
BSpec: 54369
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708211827.288601-6-jose.souza@intel.com