video: fbdev: imxfb: Convert request_mem_region + ioremap to devm_ioremap_resource

This has several advantages:

 - No need for manual undo of the two functions in the error path and
   the remove function.
 - Drops error handling in .remove()
   Note that returning early in .remove() yields resource leaks that
   often result in access of freed memory or unmapped registers later.
 - Fixes a resource leak
   request_mem_region allocates memory for the returned pointer that was
   never freed.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Helge Deller <deller@gmx.de>