drm/exynos: fimd: fix alpha setting for XR24 pixel format

XR24 planes were not shown properly, so now set the right registers
to correctly enable displaying these planes.

It also moves the alpha register settings to fimd_win_set_pixfmt()
to keep all pixel format stuff together.

v2: remove leftover var alpha

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>