video: fbdev: cirrusfb: Fix kernel-doc and set but not used warnings

Fix warnings:
- drop kernel-doc for the two debug functions to avoid the warnings
- delete unused code

v2:
  - Updated subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Thomas Zimemrmann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-28-sam@ravnborg.org