drm/debugfs: fix plain echo to connector "force" attribute

Using plain echo to set the "force" connector attribute fails with
-EINVAL, because echo appends a newline to the output.

Replace strcmp with sysfs_streq to also accept strings that end with a
newline.

v2: use sysfs_streq instead of stripping trailing whitespace

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170817104307.17124-1-m.tretter@pengutronix.de