drm/i915: Fix gen3 self-refresh watermarks

This regression has been introduced in

commit 4fe8590a
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Wed Sep 4 18:25:22 2013 +0300

    drm/i915: Use adjusted_mode appropriately when computing watermarks

I guess we should renable the enabled local variable into something a
notch more descriptive, but that's something for -next.

The effect on my i945gme netbook is pretty severe amounts of underruns
- usually the very first pixel gets used for the entire screeen.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>