drm/i915: DFSM pipe disable is valid from gen9 onwards (v2)

It's not just GEN9 platforms that allow for pipes to be disabled via
the DFSM register, but all later platforms as well.

v2: drop pointless parentheses (Ville)

Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181211192545.140081-1-bob.j.paauwe@intel.com