drm: Add and export function drm_fb_cma_sync_non_coherent

This function can be used by drivers that use damage clips and have
CMA GEM objects backed by non-coherent memory. Calling this function
in a plane's .atomic_update ensures that all the data in the backing
memory have been written to RAM.

v3: - Only sync data if using GEM objects backed by non-coherent memory.
    - Use a drm_device pointer instead of device pointer in prototype

v5: - Rename to drm_fb_cma_sync_non_coherent
    - Invert loops for better cache locality
    - Only sync BOs that have the non-coherent flag
    - Move to drm_fb_cma_helper.c to avoid circular dependency

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210523170415.90410-3-paul@crapouillou.net