drm/i915: Don't use the second dbuf slice on icl

drm/i915: Don't use the second dbuf slice on icl

The code managing the dbuf slices is borked and needs some
real work to fix. In the meantime let's just stop using the
second slice.

v2: Drop the change to intel_enabled_dbuf_slices_num() (Mahesh)

Cc: Mahesh Kumar <mahesh1.sh.kumar@gmail.com>
Reviewed-by: Imre Deak <imre.deak@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190130155110.12918-1-ville.syrjala@linux.intel.com
Reviewed-by: Mahesh Kumar <mahesh1.sh.kumar@gmail.com>