fbdev: mxsfb: use framebuffer_alloc in the correct way

framebuffer_alloc allocated the fb_info struct plus the extra
par and set fb_info->par pointer equal to this extra par. We
can refer the mxcfb_info from fb_info->par

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Tested-by: Stefan Agner <stefan@agner.ch>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>