drm/fbdev-client: Unexport drm_fbdev_client_setup()

DRM drivers invoke drm_client_setup() to set up in-kernel clients.
No driver should call drm_fbdev_client_setup() directly. Therefore,
unexport the symbol and move the declaration to the internal header
drm_client_internal.h.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241108154600.126162-4-tzimmermann@suse.de