drm: Remove drm_fbdev_cma_set_suspend()

drm_fbdev_cma_set_suspend() is not getting called from
any other places. If there is no plan to use it in future
we can remove this API.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180730192326.GA31354@jordon-HP-15-Notebook-PC