drm/i915/gen9: Use a bitmask to track dirty pipe watermarks

Slightly easier to work with than an array of bools.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-14-git-send-email-matthew.d.roper@intel.com