drm/msm/hdmi: Clean up connector gpio usage

Make gpio allocation and usage iterative by parsing the gpios on a given
platform from a list. This gives us flexibility over what all gpios exist
for a platform, whether they are input or output, and what value they
should be set to.

In particular, this will make HDMI on 8x96 platforms easier to integrate
with the driver, as it doesn't have a HPD gpio input to them. Also, it
cleans things up a bit.

We still use the legacy gpio api here, as we might need to backport this
driver to downstream kernels.

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