drm/dp: Don't zero PWMGEN_BIT_COUNT when driver_pwm_freq_hz not specified

The function drm_edp_backlight_init() is defined such that the
"driver_pwm_freq_hz" parameter is optional--it's 0 if you don't want
to futz with it. If you follow this variable through, you'll find out
that if it's 0 we won't ever set the "bl->pwmgen_bit_count", leaving
it as 0.

That means that before using it in drm_edp_backlight_enable() we need
to check to see if it's non-zero.

Programming this field to zero was confusing the panel I tested with
(Samsung ATNA33XC20). The backlight still worked somewhat but the
brightness values didn't correspond to what they should have been.

Fixes: 867cf9cd ("drm/dp: Extract i915's eDP backlight code into DRM helpers")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730084534.v2.1.If017efce7116ae8ba015ed7def840c390a0e0c77@changeid

drivers/gpu/drm/drm_dp_helper.c

@@ -3229,11 +3229,13 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
 		new_dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
 
+		if (bl->pwmgen_bit_count) {
 			ret = drm_dp_dpcd_writeb(aux, DP_EDP_PWMGEN_BIT_COUNT, bl->pwmgen_bit_count);
 			if (ret != 1)
 				drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux pwmgen bit count: %d\n",
 					    aux->name, ret);
 		}
+	}
 
 	if (bl->pwm_freq_pre_divider) {
 		ret = drm_dp_dpcd_writeb(aux, DP_EDP_BACKLIGHT_FREQ_SET, bl->pwm_freq_pre_divider);