drm: track CEA version number if present

Drivers need to know the CEA version number in addition to other display
info (like whether the display is an HDMI sink) before enabling certain
features.  So track the CEA version number in the display info
structure.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>