drm/i915/tv: Only poll for TV connections

As a probe for a TV connection modifies the TV_CTL register, it causes a
loss of sync and a regular glitch on the output. This is highly
undesirable when using the TV, so only poll for TV connections and wait
for an explicit query for detecting the disconnection event.

Reported-by: Mathew McKernan <matmckernan@rauland.com.au>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=35977
Signed-off-by: Mathew McKernan <matmckernan@rauland.com.au>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>