drm/i915/backlight: Fix divide by 0 error in i9xx_set_backlight

pwm_level_max maybe 0 we do throw a warning but move ahead with
execution which may later cause a /0 error.

--v2
-return if the warn_on gets hit [Jani]

Signed-off-by: Suraj Kandpal <[e-mail]>
Reviewed-by: Jani Nikula <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/i915/display/intel_backlight.c

···
 	struct intel_panel *panel = &connector->panel;
 	u32 tmp, mask;
 
-	drm_WARN_ON(display->drm, panel->backlight.pwm_level_max == 0);
+	if (drm_WARN_ON(display->drm, panel->backlight.pwm_level_max == 0))
+		return;
 
 	if (panel->backlight.combination_mode) {
 		struct pci_dev *pdev = to_pci_dev(display->drm->dev);