drm/i915/tv: Fix modeset flickering introduced in 7f58aabc

The tidy ups in 7f58aabc ("drm/i915:
cleanup per-pipe reg usage") changed intel_crtc->plane to intel_crtc->pipe in
intel_tv_mode_set(). This caused the screen to quickly turn off before
returning whenever modesetting/mode probing took place on my 915GM EeePC
900 creating a flickering effect. This patch changes intel_crtc->pipe back
to intel_crtc->plane which solves the problem for me.

References: https://bugs.freedesktop.org/show_bug.cgi?id=35903


Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Humbly-acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>