drm/i915: Make dirty_pipes refer to pipes

Despite the its name dirty_pipes refers to crtc indexes. Let's
change its behaviout to match the name.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011200949.7839-4-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>