drm/vc4: Allow interlaced HDMI modes from FKMS.

Having checked the firmware handling for interlaced modes,
it appears to be possible to support interlaced modes on
HDMI without adverse side effects, so do so.

https://github.com/raspberrypi/linux/issues/3694



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>