drm/i915: Use adjusted_mode in the fastboot hack to disable pfit

When booting with i915.fastboot=1, we always take tha code path and end
up undoing what we're trying to do with adjusted_mode.

Hopefully, as the fastboot hardware readout code is using adjusted_mode
as well, it should be equivalent.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>