drm/gma500: fix error check

Reviewing this block of code in cdv_intel_dp_init()

ret = cdv_intel_dp_aux_native_read(gma_encoder, DP_DPCD_REV, ...

cdv_intel_edp_panel_vdd_off(gma_encoder);
if (ret == 0) {
	/* if this fails, presume the device is a ghost */
	DRM_INFO("failed to retrieve link info, disabling eDP\n");
	drm_encoder_cleanup(encoder);
	cdv_intel_dp_destroy(connector);
	goto err_priv;
} else {

The (ret == 0) is not strict enough.
cdv_intel_dp_aux_native_read() returns > 0 on success
otherwise it is failure.

So change to <=

Fixes: d112a816 ("gma500/cdv: Add eDP support")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200805205911.20927-1-trix@redhat.com

drivers/gpu/drm/gma500/cdv_intel_dp.c

@@ -2078,7 +2078,7 @@ cdv_intel_dp_init(struct drm_device *dev, struct psb_intel_mode_device *mode_dev
 					       intel_dp->dpcd,
 					       sizeof(intel_dp->dpcd));
 		cdv_intel_edp_panel_vdd_off(gma_encoder);
-		if (ret == 0) {
+		if (ret <= 0) {
 			/* if this fails, presume the device is a ghost */
 			DRM_INFO("failed to retrieve link info, disabling eDP\n");
 			drm_encoder_cleanup(encoder);