video: fbdev: sis: Fix set but not used warnings in sis_main

Fix warnings by dropping unused variable and the
unused assignments.

v2:
  - Update subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-11-sam@ravnborg.org