drm/tegra: Changes for v4.15-rc1

This contains a bit of cleanup and some minor fixes for the host1x and
Tegra DRM drivers. There's also some more preparatory work for Tegra186
support which I'm not quite ready to send upstream because the GPIO
driver needed for HDMI support has been stuck for months, and we can't
do much without it. Hopefully that driver will land in v4.15, which
would mean we could go ahead with Tegra186 display support in v4.16.
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