video: fbdev: sis: Drop useless call to SiS_GetResInfo()

Coverity reported:

    Useless call (USELESS_CALL) side_effect_free: Calling
     SiS_GetResInfo(SiS_Pr, ModeNo, ModeIdIndex) is only useful for its
    return value, which is ignored.

And this is correct - so drop the call.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reported-by: Colin Ian King <colin.king@canonical.com>
Addresses-Coverity: ("Useless call")
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20201206190247.1861316-14-sam@ravnborg.org