drm/tegra: Implement race-free hotplug detection

A race condition currently exists on Tegra, where it can happen that a
monitor attached via HDMI isn't detected during the initial FB helper
setup, but the hotplug event happens too early to be processed by the
poll helpers because they haven't been initialized yet. This happens
because on some boards the HDMI driver can control the regulator that
supplies the +5V pin on the HDMI connector. Therefore depending on the
timing between the initialization of the HDMI driver and the rest of
DRM, it's possible that the monitor returns the hotplug signal right
within the window where we would miss it.

Unfortunately, drm_kms_helper_poll_init() will wreak havoc when called
before at least some parts of the FB helpers have been set up.

This commit fixes this by splitting out the minimum of initialization
required to make drm_kms_helper_poll_init() work into a separate
function that can be called early. It is then safe to move all of the
poll helper initialization to an earlier point in time (before the
HDMI output driver has a chance to enable the +5V supply). That way if
the hotplug signal is returned before the initial FB helper setup, the
monitor will be forcefully detected at that point, and if the hotplug
signal is returned after that it will be properly handled by the poll
helpers.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>