omapfb: dss: Fix max fclk divider for omap36xx

The drm/omap driver was fixed to correct an issue where using a
divider of 32 breaks the DSS despite the TRM stating 32 is a valid
number.  Through experimentation, it appears that 31 works, and
it is consistent with the value used by the drm/omap driver.

This patch fixes the divider for fbdev driver instead of the drm.

Fixes: f76ee892 ("omapfb: copy omapdss & displays for omapfb")
Cc: <stable@vger.kernel.org> #4.5+
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Rob Clark <robdclark@gmail.com>
[b.zolnierkie: mark patch as applicable to stable 4.5+ (was 4.9+)]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200630182636.439015-1-aford173@gmail.com

drivers/video/fbdev/omap2/omapfb/dss/dss.c

@@ -836,7 +836,7 @@ static const struct dss_features omap34xx_dss_feats = {
 };
 
 static const struct dss_features omap3630_dss_feats = {
-	.fck_div_max		=	32,
+	.fck_div_max		=	31,
 	.dss_fck_multiplier	=	1,
 	.parent_clk_name	=	"dpll4_ck",
 	.dpi_select_source	=	&dss_dpi_select_source_omap2_omap3,