drm/i915: Early rejection of buffer allocations larger than RAM

We currently try to pin and allocate the whole buffer at a time. If that
object is larger than RAM, we will try to pin the whole of physical
memory, force the machine into oom, and then still fail the allocation.

If the request is obviously too large, error out early. We opt to do
this in the backend to make it easy to use alternate paths that do not
require the entire object pinned, or may easily handle proxy objects
that are larger than physical memory.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180903083337.13134-4-chris@chris-wilson.co.uk