drm/i915: Don't leave old junk in ilk active watermarks on readout

drm/i915: Don't leave old junk in ilk active watermarks on readout

When we read out the watermark state from the hardware we're supposed to
transfer that into the active watermarks, but currently we fail to any
part of the active watermarks that isn't explicitly written. Let's clear
it all upfront.

Looks like this has been like this since the beginning, when I added the
readout. No idea why I didn't clear it up.

Cc: Matt Roper <matthew.d.roper@intel.com>
Fixes: b83515aca2c5 ("drm/i915: Init HSW watermark tracking in intel_modeset_setup_hw_state()")
Cc: stable@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463151318-14719-2-git-send-email-ville.syrjala@linux.intel.com
(cherry picked from commit 15606534bf0a65d8a74a90fd57b8712d147dbca6)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>