drm/i915: Clean up vlv_program_watermarks()

Add small helpers to make the intent of the staggered enable/disable
sequence in vlv_program_watermarks() easier on the eyes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-15-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>