drm/i915: Remove skl_ddl_allocation struct

Current consensus that it is redundant as
we already have skl_ddb_values struct out there,
also this struct contains only single member
which makes it unnecessary.

v2: As dirty_pipes soon going to be nuked away
    from skl_ddb_values, evacuating enabled_slices
    to safer in dev_priv.

v3: Changed "enabled_slices" to be "enabled_dbuf_slices_num"
    (Matt Roper)

v4: - Wrapped the line getting number of dbuf slices(Matt Roper)
    - Removed indeed redundant skl_ddb_values declaration(Matt Roper)

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200202230630.8975-2-stanislav.lisovskiy@intel.com