drm/kms: start adding command line interface using fb.

[note this requires an fb patch posted to linux-fbdev-devel already]

This uses the normal video= command line option to control the kms
output setup at boot time. It is used to override the autodetection
done by kms.

video= normally takes a framebuffer as the first parameter, in kms
it will take a connector name, DVI-I-1, or LVDS-1 etc. If no output
connector is specified the mode string will apply to all connectors.

The mode specification used will match down the probed modes, and if
no mode is found it will add a CVT mode that matches.

video=1024x768 - all connectors match a 1024x768 mode or add a CVT on
video=VGA-1:1024x768, VGA-1 connector gets mode only.

The same strings as used in current fb modedb.c are used, except I've
added three more letters, e, D, d, e = enable, D = enable Digital,
d = disable, which allow a connector to be forced into a certain state.

Signed-off-by: Dave Airlie <airlied@redhat.com>