drm/i915: Remove redundant clear of appgtt

Upon creation of the va range, it is initialised to point at scratch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-10-chris@chris-wilson.co.uk