drm/i915: Clear TX FIFO reset master override bits on chv

Clear the override bits to make sure the hardware manages
the TX FIFO reset master on its own.

v2: Squash with the earlier attempt at forcing the override bits

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>