drm/i915: Also clear the punit's PDATA sideband register

As the previous punit i/o may have failed, the contents of the PDATA are
undefined. Always clear it to 0 prior to sending the command.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125134808.6152-2-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>