drm/i915/dg2: Add dbuf programming

DG2 extends our DDB to four DBuf slices; pipes A+B only have access to
the first two slices, whereas pipes C+D only have access to the second
two.

Confusingly, our bspec decided to switch from 1-based numbering
of dbuf slices (S1, S2) to 0-based numbering (S0, S1, S2, S3) in
Display13.  At the moment we're using the 0-based number scheme for the
DBUF_CTL_S() register addressing, but the 1-based number scheme in the
actual slice assignment tables.  We may want to consider switching the
assignment over to 0-based numbering too at some point...

Bspec: 49255
Bspec: 50057
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721223043.834562-16-matthew.d.roper@intel.com