video: fbdev: tridentfb: remove deadcode on unreachable case statement

The value of tmp being used in the switch statement has the range of
just 0..3 hence the case 4 statement can never be reached and is
deadcode and can be removed.

Detected by CoverityScan, CID#744384 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>