video: fbdev: imxfb: fix sparse warnings about using incorrect types

Use ->screen_buffer instead of ->screen_base to fix sparse warnings.

[ Please see commit 17a7b0b4 ("fb.h: Provide alternate screen_base
  pointer") for details. ]

Reported-by: kbuild test robot <lkp@intel.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: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>