drm/vgem: fix cache synchronization on arm/arm64

drm_cflush_pages() is no-op on arm/arm64.  But instead we can use
dma_sync API.

Fixes failures w/ vgem_test.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190717211542.30482-1-robdclark@gmail.com