drm: encoder_slave: fix refcouting error for modules

module_put() balances try_module_get(), not request_module(). Fix the
error path to match that.

Fixes: 2066facc ("drm/kms: slave encoder interface.")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Wolfram Sang <wsa@kernel.org>