drm/i915/skl: Correcting the flushing of pipe

We were incorreectly bypassing the flush everytime which led to fifo
underrun when more than one plane is enabled.

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Satheeshakrishna <M&lt;satheeshakrishna.m@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>