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

drivers/gpu/drm/drm_debugfs.c

@@ -311,13 +311,13 @@ static ssize_t connector_write(struct file *file, const char __user *ubuf,
 
 	buf[len] = '\0';
 
-	if (!strcmp(buf, "on"))
+	if (sysfs_streq(buf, "on"))
 		connector->force = DRM_FORCE_ON;
-	else if (!strcmp(buf, "digital"))
+	else if (sysfs_streq(buf, "digital"))
 		connector->force = DRM_FORCE_ON_DIGITAL;
-	else if (!strcmp(buf, "off"))
+	else if (sysfs_streq(buf, "off"))
 		connector->force = DRM_FORCE_OFF;
-	else if (!strcmp(buf, "unspecified"))
+	else if (sysfs_streq(buf, "unspecified"))
 		connector->force = DRM_FORCE_UNSPECIFIED;
 	else
 		return -EINVAL;