drm: Allow creating blob properties without copy

Make the data parameter to drm_property_create_blob optional; if
omitted, the copy will be skipped and the data will be empty.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>
Tested-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>