drm/cma-helper: Remove drm_fb_cma_debugfs_show()

drm/cma-helper: Remove drm_fb_cma_debugfs_show()

drm_fb_cma_debugfs_show() and drm_gem_cma_describe() are superseded
by drm_framebuffer_debugfs_init() and drm_gem_cma_print_info().

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107191348.17555-13-noralf@tronnes.org