drm/i915: Use the ilk_disable_lp_wm() return value

drm/i915: Use the ilk_disable_lp_wm() return value

ilk_disable_lp_wm() will tell us whether the LP1+ watermarks were
disabled or not, and hence whether we need to for the vblank wait or
not. Let's use that information to eliminate some useless vblank
waits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>