drm/amd/display: Fix possible memleak in dp_trigger_hotplug()

If parse_write_buffer_into_params() fails, we should free
wr_buf before return.

Fixes: 6f77b2ac ("drm/amd/display: Add connector HPD trigger debugfs entry")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>