drm/i915: Introduce a for_each_intel_encoder() macro

Following the established idom, let's provide a macro to iterate through
the encoders.

spatch helps, once more, for the substitution:

  @@
  iterator name list_for_each_entry;
  iterator name for_each_intel_encoder;
  struct intel_encoder * encoder;
  struct drm_device * dev;
  @@
  -list_for_each_entry(encoder, &dev->mode_config.encoder_list, base.head) {
  +for_each_intel_encoder(dev, encoder) {
    ...
  }

I also modified a few call sites by hand where a pointer to mode_config
was directly used (to avoid overflowing 80 chars).

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Wrap paramters correctly in the macro and remove spurious
space checkpatch noticed.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>