omapfb: use devm_platform_ioremap_resource_byname

Use the devm_platform_ioremap_resource_byname() helper instead of
calling platform_get_resource_byname() and devm_ioremap_resource()
separately.

Signed-off-by: Qilong Zhang <zhangqilong3@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
[fixed warning - unused variable]
[fixed checkpatch warning - fixed indent]
Link: https://patchwork.freedesktop.org/patch/msgid/20200916111353.105914-1-zhangqilong3@huawei.com