drm/i915: Use enum pipe instead of crtc index to track active pipes

drm/i915: Use enum pipe instead of crtc index to track active pipes

We may need to eliminate the crtc->index == pipe assumptions from
the code to support arbitrary pipes being fused off. Start that by
switching some bitmasks over to using pipe instead of the crtc index.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821173033.24123-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>