drm: Drop locking cargo-cult from drm_mode_config_init

This is single-threaded setup code, no need for locks. And anyway,
all properties need to be set up before the driver is registered
anyway, they can't be hot-added.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213230814.19598-5-daniel.vetter@ffwll.ch