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>