drm/vram-helper: fix function names in vram helper doc

Refer to drmm_vram_helper_init() instead of the non-existent
drmm_vram_helper_alloc_mm().

Fixes: a5f23a72 ("drm/vram-helper: Managed vram helpers")
Signed-off-by: Luc Ma <luc@sietium.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/64583db2.630a0220.eb75d.8f51@mx.google.com