Revert "gpu/drm: ingenic: Add option to mmap GEM buffers cached"

This reverts commit 37054fc8 ("gpu/drm: ingenic: Add option to mmap
GEM buffers cached")

At the very moment this commit was created, the DMA API it relied on was
modified in the DMA tree, which caused the driver to break in
linux-next.

Revert it for now, and it will be resubmitted later to work with the new
DMA API.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201004141758.1013317-1-paul@crapouillou.net