drm/i915: Determine uses-full-ppgtt from context for execbuf

Rather than inspect the global module parameter for whether full-ppgtt
maybe enabled, we can inspect the context directly as to whether it has
its own vm.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180901092451.7233-1-chris@chris-wilson.co.uk