drm: add bit depth parsing

EDID 1.4 digital monitors report the bit depth supported in the input
field.  Add support for parsing this out and storing the info in the
display_info structure for use by drivers.

[airlied: tweaked to fix inter-patch dependency]
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>