arm: plat-omap: iommu: fix request_mem_region() error path

request_mem_region() error exit will leak ioremapped memory. Fix this
by moving the ioremap() after request_mem_region(), which is the proper
order to do this anyway.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>