drm/i915/ringbuffer: Write the value passed in to the tail register

drm/i915/ringbuffer: Write the value passed in to the tail register

This should fix the error along the reset path were we tried to clear the
tail register by setting it to 0, but were in fact setting it to the
current value and complaining when it did not reset to 0.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>