Fix warning by deleting the variable. The function call
viafb_get_best_mode() was verified to have no side-effects,
and thus could be dropped too.

v2:
  - Update subject (Lee)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-12-sam@ravnborg.org