drm/i915: fix fastboot pfit disable hack to update pipe w/h

When fastbooting, we read out the pipe timings early on, and then in a
panel fitted config, disable the fitter later.  But we weren't updating
the pipe src h/w, which meant the mouse cursor was clipped to the
pfitted size rather than the native size set later.  Fix that up so the
cursor is visible in the new mode.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>