drm/client: Fix: drm_client_new: Don't require DRM to be registered

Commit 894a677f ("drm/cma-helper: Use the generic fbdev emulation")
broke almost all drivers that use the CMA helper.

The reason is that drm_client_new() requires that the DRM device has
been registered, but the drivers register fbdev before registering DRM.

Remove the requirement that DRM should be registered when creating a
new client.

Fixes: c76f0f7c

 ("drm: Begin an API for in-kernel clients")
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Icenowy Zheng <icenowy@aosc.io>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180711155632.37437-1-noralf@tronnes.org