drm/i915: Introduce for_each_dbuf_slice_in_mask macro

We quite often need now to iterate only particular dbuf slices
in mask, whether they are active or related to particular crtc.

v2: - Minor code refactoring
v3: - Use enum for max slices instead of macro

Let's make our life a bit easier and use a macro for that.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200519131117.17190-6-stanislav.lisovskiy@intel.com