drm/i915: Drop intel_update_sprite_watermarks

The only platform that still has an update_sprite_wm entrypoint is SKL;
on SKL, intel_update_sprite_watermarks just updates intel_plane->wm and
then performs a regular watermark update.  However intel_plane->wm is
only used to update a couple fields in intel_wm_config, and those fields
are never used by the SKL code, so on SKL an update_sprite_wm is
effectively identical to an update_wm call.  Since we're already
ensuring that the regular intel_update_wm is called any time we'd try to
call intel_update_sprite_watermarks, the whole call is redundant and can
be dropped.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Smoke-tested-by: Paulo Zanoni <przanoni@gmail.com>
Link: http://patchwork.freedesktop.org/patch/60372/