drm/i915: Unify the low level dbuf code

The low level dbuf slice code is rather inconsitent with its
functiona naming and organization. Make it more consistent.

Also share the enable/disable functions between all platforms
since the same code works just fine for all of them.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225171125.28885-8-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>