video: fbdev: controlfb: add COMPILE_TEST support

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

While at it:
- convert driver to use eieio() and dcbf() helpers instead of
  open-coding them
- add invalid_vram_cache() helper to avoid code duplication

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200324134508.25120-3-b.zolnierkie@samsung.com