drm/i915: Allow !join_mbus cases for adlp+ dbuf configuration

Reintroduce the !join_mbus single pipe cases for adlp+.

Due to the mbus relative dbuf offsets in PLANE_BUF_CFG we
need to know the actual slices used by the pipe when doing
readout, even when mbus joining isn't enabled. Accurate
readout will be needed to properly sanitize invalid BIOS
dbuf configurations.

This will also make it much easier to play around with the
!join_mbus configs for testin/workaround purposes.

Cc: <stable@vger.kernel.org> # v5.14+
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220204141818.1900-1-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>