drm/i915/tv: Use the scanline counter for timestamps on i965gm TV output

drm/i915/tv: Use the scanline counter for timestamps on i965gm TV output

Just like the frame counter, the pixel counter also reads zero
all the time when the TV encoder is used. Fortunately the
scanline counter still works sufficiently well so let's use that
to correct the vblank timestamps. Otherwise the timestamps may
en up out of whack, and since we use them to guesstimate the
vblank counter value that may end up incorrect as well.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125181931.19482-2-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>