The computation of required framebuffer size in

commit d978ef14
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Mar 7 08:57:51 2014 -0800

    drm/i915: Wrap the preallocated BIOS framebuffer and preserve for KMS fbcon v12

is too optimistic, and would rely on the invariant fb being
reconstructed to exactly fit each pipe (and probably ignore hardware
limits). Instead, we want to compute the upper bound on what the display
engine will access and ensure that is within the inherited framebuffer.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>