drm/i915: overlay on gen2 can't address above 1G

drm/i915: overlay on gen2 can't address above 1G

So set the coherent dma mask accordingly. This dma mask is only used
for physical objects, so it won't really matter allocation-wise.

Now this never really surfaced because sane 32bit kernels only have 1G
of lowmem. But some eager testers (distros?) still carry around the patch
to adjust lowmem via a kconfig option. And the kernel seems to favour
high allocations on boot-up, hence the overlay blowing up reliably.

Because the patch is tiny and nicely shows how broken gen2 is it's imho
worth to merge despite the fact that mucking around with the lowmem/
highmem division is (no longer) supported.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=28318
Cc: stable@kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>