drm/i915: Use the correct scanout alignment for fbcon.

This fixes a potential modesetting error during boot with plymouth on
Broadwater and Crestline introduced with 9df47c. The framebuffer was
hard-coding an alignment of 64K, but the modesetting code required the
documented alignment of 128K. The result was that we would attempt to
unbind the pinned fbcon buffer, triggering an ERROR and ultimately
failing the mode change.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>