fbdev: da8xx-fb: use devm_platform_ioremap_resource()

Shrink the code a bit by using the new helper wrapping the calls to
platform_get_resource() and devm_ioremap_resource() together.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>