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.