drm/i915: prefer for_each_intel_* macros for iteration

Use the for_each_intel_* macros for iterating intel_encoder,
intel_connector, and intel_crtc. No functional changes.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450262896-5325-1-git-send-email-jani.nikula@intel.com