video: fbdev: w100fb: add COMPILE_TEST support

Add COMPILE_TEST support to w100fb 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/a929db5e-d373-7b09-ae2b-efec227f7e85@samsung.com