drm/vc4: Add DRM_MODE_FLAG_DBLCLK support to vc4-fkms

480i and several other modes use DRM_MODE_FLAG_DBLCLK and pixel
replication.

Add in flags for that so that FKMS can select CEA modes 6 & 7.

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