drm/amd/display: Check hpd_gpio for NULL before accessing it

dal_gpio_open and dal_gpio_unlock_pin dereference hpd_gpio.
Check for NULL before calling those functions.

Fixes: ac627caf6b9275a ("drm/amd/display: add gpio lock/unlock")
Reported-by: Przemek Socha <soprwa@gmail.com>
CC: Chiawen Huang <chiawen.huang@amd.com>
CC: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>