fbdev: imsttfb: fix a resource leak in probe

I've re-written the error handling but the bug is that if init_imstt()
fails we need to call iounmap(par->cmap_regs).

Fixes: c75f5a55

 ("fbdev: imsttfb: Fix use after free bug in imsttfb_probe")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Helge Deller <deller@gmx.de>