drm/i915: Fix the computation of required fb size for pipe

drm/i915: Fix the computation of required fb size for pipe

The computation of required framebuffer size in

commit 57291828cf5a7ca1b07acfd940ae43cde6698f2f
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>