drm: Link CMA framebuffer helpers into KMS helper library

Linking the CMA framebuffer helpers into a CMA helper library in
commit 4b2b5e14

 ("drm: Move GEM memory managers into modules")
results in linker errors:

  arm-linux-gnueabihf-ld: drivers/gpu/drm/drm_fb_cma_helper.o: \
	  in function `drm_fb_cma_get_gem_obj': \
	  drivers/gpu/drm/drm_fb_cma_helper.c:46: undefined reference \
	  to `drm_gem_fb_get_obj'
  arm-linux-gnueabihf-ld: drivers/gpu/drm/drm_fb_cma_helper.c:46: \
	  undefined reference to `drm_gem_fb_get_obj'
  arm-linux-gnueabihf-ld: drivers/gpu/drm/drm_fb_cma_helper.c:46: \
	  undefined reference to `drm_gem_fb_get_obj'
  arm-linux-gnueabihf-ld: drivers/gpu/drm/drm_fb_cma_helper.o: in \
	  function `drm_fb_cma_sync_non_coherent': \
	  drivers/gpu/drm/drm_fb_cma_helper.c:133: undefined reference \
	  to `drm_atomic_helper_damage_iter_init'
  arm-linux-gnueabihf-ld: drivers/gpu/drm/drm_fb_cma_helper.c:135: \
	  undefined reference to `drm_atomic_helper_damage_iter_next'

Link the CMA framebuffer helpers into the KMS helper library to
fix the problem.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 4b2b5e14

 ("drm: Move GEM memory managers into modules")
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20211026175700.27664-1-tzimmermann@suse.de