video: fbdev: mx3fb: Fix kernel-doc, set but not used and string warnings

Fix W=1 warnings:
- Fix kernel-doc
- Drop unused code/variables
- Use memcpy to copy a string without zero-termination
  strncpy() generates a warning

v2:
  - Updated subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Xiaofei Tan <tanxiaofei@huawei.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-19-sam@ravnborg.org