Add gamma-drm, works under TTY, requires membership of the video group

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Beginning on full multimonitor, multiscreen support, implemented in gamma-drm, other parts does not compile yet

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Fix ENABLE_DRM check to say ENABLE_DRM and not ENABLE_RANDR

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Be helpful about screens versus CRTC:s: suggest CRTC if only screen 0 exists, and suggest randr if using vidmode in such case

Signed-off-by: Mattias Andrée <maandree@operamail.com>