drm/omap: fix NULL but dereferenced coccicheck error

Fix the following coccicheck warning:
./drivers/gpu/drm/omapdrm/omap_overlay.c:89:22-25: ERROR: r_ovl is NULL
but dereferenced.

Here should be ovl->idx rather than r_ovl->idx.

Fixes: e02b5cc9 ("drm/omap: Add a 'right overlay' to plane state")
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220307095612.409090-1-wanjiabing@vivo.com