drm/i915: Use a sentinel to terminate the dbuf slice arrays

Make life a bit simpler by sticking a sentinel at the end of
the dbuf slice arrays. This way we don't need to pass in the
size. Also unify the types (u8 vs. u32) for active_pipes.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225171125.28885-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>