drm/mediatek: dp: drm_err => dev_err in HPD path to avoid NULL ptr

The function mtk_dp_wait_hpd_asserted() may be called before the
`mtk_dp->drm_dev` pointer is assigned in mtk_dp_bridge_attach().
Specifically it can be called via this callpath:
- mtk_edp_wait_hpd_asserted
- [panel probe]
- dp_aux_ep_probe

Using "drm" level prints anywhere in this callpath causes a NULL
pointer dereference. Change the error message directly in
mtk_dp_wait_hpd_asserted() to dev_err() to avoid this. Also change the
error messages in mtk_dp_parse_capabilities(), which is called by
mtk_dp_wait_hpd_asserted().

While touching these prints, also add the error code to them to make
future debugging easier.

Fixes: 7eacba9a083b ("drm/mediatek: dp: Add .wait_hpd_asserted() for AUX bus")
Signed-off-by: Douglas Anderson <[e-mail]>
Reviewed-by: CK Hu <[e-mail]>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250116094249.1.I29b0b621abb613ddc70ab4996426a3909e1aa75f@changeid/
Signed-off-by: Chun-Kuang Hu <[e-mail]>

drivers/gpu/drm/mediatek/mtk_dp.c

···
 
 	ret = drm_dp_dpcd_readb(&mtk_dp->aux, DP_MSTM_CAP, &val);
 	if (ret < 1) {
-		drm_err(mtk_dp->drm_dev, "Read mstm cap failed\n");
+		dev_err(mtk_dp->dev, "Read mstm cap failed: %zd\n", ret);
 		return ret == 0 ? -EIO : ret;
 	}
 
···
 					DP_DEVICE_SERVICE_IRQ_VECTOR_ESI0,
 					&val);
 		if (ret < 1) {
-			drm_err(mtk_dp->drm_dev, "Read irq vector failed\n");
+			dev_err(mtk_dp->dev, "Read irq vector failed: %zd\n", ret);
 			return ret == 0 ? -EIO : ret;
 		}
 
···
 
 	ret = mtk_dp_parse_capabilities(mtk_dp);
 	if (ret) {
-		drm_err(mtk_dp->drm_dev, "Can't parse capabilities\n");
+		dev_err(mtk_dp->dev, "Can't parse capabilities: %d\n", ret);
 		return ret;
 	}