drm/ingenic: Fix bad revert

Fix a badly reverted commit. The revert commit was cherry-picked from
drm-misc-next to drm-misc-next-fixes, and in the process some unrelated
code was added.

Fixes: a3fb64c0 ("Revert "gpu/drm: ingenic: Add option to mmap GEM buffers cached"")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201012102509.10690-1-paul@crapouillou.net