drm/mxsfb: drop unused function parameter

flags is unused since the driver was introduced in commit 45d59d70
("drm: Add new driver for MXSFB controller").

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716174139.16602-1-u.kleine-koenig@pengutronix.de