Revert "drm/i915: Do not allow buffers at offset 0"

Revert "drm/i915: Do not allow buffers at offset 0"

This reverts commit 78572d39317c6b01885a9a4f5d4074c0a02c538c.

The patch completely lacks a detailed explanation of what exactly
blows up and how, so is insufficiently justified as a band-aid.

Otoh the justification as a safety measure against userspace botching
up relocations is also fairly weak: If we want real project we need to
at least make the gab big enough that the gpu doesn't scribble over
more important stuff. With 4k screens that would be 32MB.

Also I think this would be much better in conjunction with a (debug)
switch to disable our use of the scratch page.

Hence revert this.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>