drm/i915: Fix the level 0 max_wm hack on VLV/CHV

The watermark should never exceed the FIFO size, so we need to
check against the current FIFO size instead of the theoretical
maximum when we clamp the level 0 watermark.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-4-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>