drm: unexport drm_ioctl_permit

Since the last user of drm_ioctl_permit was removed, and it's now only
used in drm_ioctl.c, unexport the symbol.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210813085450.32494-3-desmondcheongzx@gmail.com