drm/i915/gen12: MBUS B credit change

drm/i915/gen12: MBUS B credit change

Previously, the recommended B credit for all platforms was 24 / number
of pipes, which would give 6 for newer platforms with 4 pipes. However 6
is not enough and we need 12 on these cases.

We also need a different BW credit for these platforms.

Cc: Arthur J Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190711173115.28296-17-lucas.demarchi@intel.com