video: fbdev: arcfb: add COMPILE_TEST support

Add COMPILE_TEST support to arcfb 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/acf2cc2e-614d-f0fb-ce40-cee62bfcde4c@samsung.com