drm: fix race between driver loading and userspace open.

Not 100% sure this is due to BKL removal, its most likely a combination
of that + userspace timing changes in udev/plymouth. The drm adds the sysfs
device before the driver has completed internal loading, this causes udev
to make the node and plymouth to open it before we've completed loading.

The proper solution is to delay the sysfs manipulation until later in loading
however this causes knock on issues with sysfs connector nodes, so we can use
the global mutex to serialise loading and userspace opens.

Reported-by: Toni Spets (hifi on #radeon)
Signed-off-by: Dave Airlie <airlied@redhat.com>