drm/kms: rip out drm_mode_connector_detach_encoder

It's only used by imx, and that one gets it wrong - there's no need
to deteach the encoder before removing it.

And really, neither current drm modesetting code nor all the userspace
we have can handle dynamic changes in the set of possible encoders for
a given connector. So let's just remove this before someone starts
doing something really nasty with it.

As a plus, one less kerneldoc comment to write.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>