OMAP: DSS2: remove non-zero check on variable r

Variable r is being initialized to zero, so the check of a non-zero
rv is redundant and can be removed.

It appears that the previous case statements set r to be -EINVAL
and the "Fallthrough" comment afterwards suggested it was going
to fall through to this non-zero check but won't because of the
break statement. Remove the confusion by removing the Fallthrough
comment too.

Addresses-Coverity: ("Logically dead code")
Fixes: b39a982d ("OMAP: DSS2: omapfb driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204152847.1435188-1-colin.king@canonical.com