Fixes gcc '-Wunused-but-set-variable' warning:

drivers/video/fbdev/mxsfb.c: In function 'mxsfb_restore_mode':
drivers/video/fbdev/mxsfb.c:633:11: warning:
 variable 'line_count' set but not used [-Wunused-but-set-variable]

It's never used since introduction in commit f0a523b5 ("video: Add i.MX23/28
framebuffer driver") and can be removed.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Melchior Franz <melchior.franz@ginzinger.com>
[b.zolnierkie: minor patch summary fixup]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>