drm/i915: Set preferred mode for integrated TV according to TV format

In order to get best possible quality image we chose 640x480 for
NTSC, PAL and 480p, 1280x720 for 720p, 1920x1080 for 1080i/p
TV format respectively.

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>