omapfb/dss: remove unneeded conversions to bool

Found with scripts/coccinelle/misc/boolconv.cocci.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016180424.23907-1-afd@ti.com