drm/vram-helper: Unexport drm_vram_helper_{alloc,release}_mm()

All GEM-VRAM-based drivers use auto-cleanup via drmm_vram_helper_init().
Unexport the manual APIs and make them internal implementation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702075434.27677-4-tzimmermann@suse.de