drm/i915: Nuke skl wm.dirty_pipes bitmask

The dirty_pipes bitmask is now unused. Get rid of it.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120174728.21095-4-ville.syrjala@linux.intel.com