drm/i915: Use a crtc mask instead of a refcount for dpll functions, v2.

drm/i915: Use a crtc mask instead of a refcount for dpll functions, v2.

This makes it easier to verify correct dpll setup with only a single crtc.
It is also useful to detect double dpll enable/disable.

Changes since v1:
- Rebase on top of Ander's dpll rework.
- Change debugfs active to a mask.
- Change enabled_crtcs and active_crtcs to unsigned.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457944075-14123-2-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>