video: fbdev: wm8505fb: add COMPILE_TEST support

Add COMPILE_TEST support to wm8505fb driver for better compile
testing coverage.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/900c16b3-9306-7d17-f467-0f98bc95416a@samsung.com