drm/msm/hdmi: Prevent gpio_free related kernel warnings

Calling the legacy gpio_free on an invalid GPIO (a GPIO numbered -1)
results in kernel warnings. This causes a lot of backtraces when
we try to unload the drm/msm module.

Call gpio_free only on valid GPIOs.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>