drm: Move client code to clients/ subdirectory

Just move some files around to keep source code well organized. Plus
fix a type in the help text of CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM. No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241108154600.126162-2-tzimmermann@suse.de