drm/vc4: drv: Add include guards

vc4_drv.h doesn't have any include guards which prevents it from being
included twice. Let's add them.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/68e89e315c4c35b313efc277c9642eca684e0ade.1590594512.git-series.maxime@cerno.tech