drm: Move GEM memory managers into modules

DRM core uses the GEM base object to access GEM functionality. It does
not depend on individual implementations. Move the code into modules.

Also move the CMA framebuffer helpers into the CMA's module, as they're
not usable without CMA.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020131941.15367-4-tzimmermann@suse.de