drm/i915/icl: Prevent incorrect DBuf enabling

Pretend that we have only 1 DBuf slice and that 1 slice is always
enabled, until we have a proper way for on-demand toggling of the second
slice.  Currently we'll try to incorrectly enable DBuf even when all
pipes are disabled and we are already runtime suspended (as the computed
number of DBuf slices will be 1 in that case).

This also means we'll leave the second slice enabled redundantly (except
when suspended), but that's an acceptable tradeoff until we have a
proper solution.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108756


Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190307103235.23538-1-imre.deak@intel.com