drm/i915/tv: Bypass the vertical filter if possible

Let's switch the pipe into interlaced mode and switch off
the TV encoder vertical filter if the pipe vdisplay
matches the TV YSIZE exactly.

While I didn't measure it I presume this might reduce
the power consumption a little bit, and the pixel rate
is halved as the pipe will now fetching in interlaced
mode rather than in progressive mode (effectively the
same difference as between IF-ID vs. PF-ID pfit modes
on more modern hardware) so a bit easier on the memory
bandwidth.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129141913.5515-2-ville.syrjala@linux.intel.com


Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>