video: fbdev: imxfb: ensure balanced regulator usage

The fbdev framework doesn't care to call the .set_power callback only on
changes. So the driver has to care for itself that the regulator doesn't
get disabled more often than enabled.

This fixes the regulator warning

	unbalanced disables for lcd supply

which can be triggered by doing

	echo 4 > /sys/class/lcd/imxfb-lcd/lcd_power

twice.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323211626.24812-1-u.kleine-koenig@pengutronix.de