fbdev: mxsfb: implement FB_PRE_INIT_FB option

The FB_PRE_INIT_FB option keeps the kernel from reinitializing the display
and prevents flickering during the transition from a bootloader splash
screen to the kernel logo screen.

Make this option available for the mxsfb driver.

Signed-off-by: Melchior Franz <melchior.franz@ginzinger.com>
Signed-off-by: Martin Kepplinger <martin.kepplinger@ginzinger.com>
Signed-off-by: Manfred Schlaegl <manfred.schlaegl@ginzinger.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>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>