drm/i915/tv: Fix adjusted_mode dotclock for interlaced modes

drm/i915/tv: Fix adjusted_mode dotclock for interlaced modes

intel_tv_mode_to_mode() assumes the pipe will be in progressive
fetch mode, and thus when programming the pipe into interlaced
mode we have to halve the calculated dotclock to get the correct
field duration.

This becomes more important when we start to program the pipe
into interlaced mode on i965gm as we depend on the timestamps
to get accurate frame counter values. Withot halving the clock
our guesstimated frame counter would tick at twice the expected
speed.

Cc: Imre Deak <imre.deak@intel.com>
Fixes: d9e05f2c6395 ("drm/i915/tv: Fix >1024 modes on gen3")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129141913.5515-1-ville.syrjala@linux.intel.com
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>