drm/i915: Introduce skl_ddb_entry_for_slices()

Generalize icl_get_first_dbuf_slice_offset() into something that
just gives us the start+end of the dbuf chunk covered by the
specified slices as a standard ddb entry. Initial idea was to use
it during readout as well, but we shall see.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122205633.18492-5-ville.syrjala@linux.intel.com