drm/cma-helper: Remove unused fbdev code

CMA helper drivers have been converted to drm_fbdev_generic_setup()
so the fbdev code can be removed.

v3: Remove CMA specific conditional in the generic fbdev client

v2: Clean up the includes some more (Laurent)

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114121059.20704-1-noralf@tronnes.org