drm: Add DRM_MODE_TV_MODE_MONOCHROME

Add this as a value for enum_drm_connector_tv_mode, represented
by the string "Mono", to generate video with standard timings
but no colour encoding or bursts. Define it to have no pedestal
(since only NTSC-M calls for a pedestal).

Change default mode creation to acommodate the new tv_mode value
which comprises both 525-line and 625-line formats.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>