drm: Remove DRM_KMS_FB_HELPER Kconfig option

It's only used by DRM_FBDEV_EMULATION, so inline it there.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210415110040.23525-5-tzimmermann@suse.de