drm/i915: Polish for_each_dbuf_slice()

drm/i915: Polish for_each_dbuf_slice()

Now that we have the dbuf slice mask stored in the device info
let's use it for for_each_dbuf_slice_in_mask*().

With this we cal also rip out intel_dbuf_size() and
intel_dbuf_num_slices().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416171011.19012-7-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>