drm/i915: Use IS_BROADWELL() instead of IS_GEN8() in forcewake code

IS_GEN8() is a bad check in the forcewake code due to bdw vs. chv
differences. Use IS_BROADWELL() instead.

The only actual bug here is that we currently call
__gen7_gt_force_wake_mt_reset() on chv. On the other places we
have checked for chv before using IS_GEN8(), but change them
to use IS_BROADWELL() anyway to reduce the chance of accidents in the
future.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>