drm/lease: Send a distinct uevent

Sending the exact same hotplug event is not great uapi. Luckily the
only already merged implementation of leases (in the -modesetting
driver) doesn't care about what kind of uevent it gets, and
unconditionally processes both hotplug and lease changes. So we can
still adjust the uapi here.

But e.g. weston tries to filter stuff, and I guess others might want
to do that too. Try to make that possible. Cc: stable since it's uapi
adjustement that we want to roll out everywhere.

Michel Dänzer mentioned on irc that -amdgpu also has lease support. It
has the same code flow as -modesetting though, so we can still go
ahead.

v2: Mention -amdgpu (Michel)

Cc: Keith Packard <keithp@keithp.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: stable@vger.kernel.org
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129094226.30591-1-daniel.vetter@ffwll.ch