drm/i915: Introduce a for_each_plane() macro

Tired of copy/pasting things around.

v2: Rebase on top of the for_each_pipe() change adding dev_priv as first
    argument.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>