V4L/DVB (7118): dvb-ttpci: Improved display of still pictures

Improved display of still pictures (VIDEO_STILLPICTURE ioctl).
Ensure that both fields are displayed for progressive frames.

Thanks to Reinhard Nissl and Klaus Schmidinger for finding out
that the FREEZE command does this.

Thanks-to: Reinhard Nissl <rnissl@gmx.de>
Thanks-to: Klaus Schmidinger <Klaus.Schmidinger@cadsoft.de>
Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>