drm/client: Add client free callback to unprepare fb_helper

drm/client: Add client free callback to unprepare fb_helper

Add free callback to struct drm_client_funcs. Invoke function to
free the client memory as part of the release process. Implement
free for fbdev emulation.

Fbdev emulation allocates and prepares client memory in
drm_fbdev_client_setup(). The release happens in fb_destroy from
struct fb_ops. Multiple implementations of this callback exist in
the various drivers that provide an fbdev implementation. Each of
them needs to follow the implementation details of the fbdev setup
code.

Adding a free callback for the client puts the unprepare and release
of the fbdev client in a single place.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com> # core, msm
Acked-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com> # omapdrm
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com> # gma500
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20251009132006.45834-2-tzimmermann@suse.de