drm/core: Add drm_for_each_encoder_mask, v2.

This is similar to the other drm_for_each_*_mask functions.

Changes since v1:
- Use for_each_if

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452160762-30487-3-git-send-email-maarten.lankhorst@linux.intel.com