drm/imx: remove drm_mode_connector_detach_encoder harder

Since the last time I've looked more of this stuff sprouted up. Stomp
it down again.

Repeating the original justification for ripping this all out: There's
absolutely no need to deteach connectors before cleaning them up at
driver unload time. And since drm doesn't support hotplugging kms
objects at all it's positively dangerous to attempt this at runtime.
Luckily imx only detachs at driver cleanup time and hence we can
savely remove this.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>