drm/vc4: Fix comment in vc4_drv.h

Fixes a copy&paste error.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1495550187-525-1-git-send-email-boris.brezillon@free-electrons.com