drm/i915: Make skl_compute_dbuf_slices() behave consistently for all platforms

drm/i915: Make skl_compute_dbuf_slices() behave consistently for all platforms

Currently skl_compute_dbuf_slices() returns 0 for any inactive pipe on
icl+, but returns BIT(S1) on pre-icl for any pipe (whether it's active or
not). Let's make the behaviour consistent and always return 0 for any
inactive pipe.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225171125.28885-6-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>