drm: Make drm_vm_open/close_locked private to drm_vm.c

It's only used for legacy mmaping support now.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-8-git-send-email-daniel.vetter@ffwll.ch