drm/i915: Initialise i915_gem_object_create_from_data() directly

Use pagecache_write to avoid shmemfs clearing the pages prior to us
immediately overwriting them with our data.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317194648.12468-2-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>