drm/i915: Remove update_sprite_watermarks.

Commit 791a32be ("drm/i915: Drop intel_update_sprite_watermarks")
removes the use of this variable, but forgot to remove it.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455108583-29227-5-git-send-email-maarten.lankhorst@linux.intel.com