video: imsttfb: check for ioremap() failures

We should check if ioremap() were to somehow fail in imsttfb_probe() and
handle the unwinding of the resources allocated here properly.

Ideally if anyone cares about this driver (it's for a PowerMac era PCI
display card), they wouldn't even be using fbdev anymore.  Or the devm_*
apis could be used, but that's just extra work for diminishing
returns...

Cc: Finn Thain <fthain@telegraphics.com.au>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210503115736.2104747-68-gregkh@linuxfoundation.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>