drm: Lighten sysfs connector 'status'

Since the beginning, sysfs/connector/status has done a heavyweight
detection of the current connector status. But no user, such as upowerd
or logind, has ever desired to initiate a probe. Move the probing into a
new attribute so that existing readers get the behaviour they desire.

v2: David Herrmann suggested using "echo detect > /sys/.../status" to
trigger the probing, which is a fine idea. This extends that to also
allow the user to apply the force detection overrides at runtime.

v3: Now with airlied's email address fixed! Requires sysfs_streq()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>