drm/i915: Reject vma creation larger than address space

Disallow creation of a vma that is larger than the available address
space, or triggers an overflow on fence expansion.

Testcase: igt/gem_exec_reloc/gtt-32
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170119192659.31789-3-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>