HDMI 1.4 adds 4 "4k x 2k" modes in the the CEA vendor specific block.

With this commit, we now parse this block and expose the 4k modes that
we find there.

v2: Fix the "4096x2160" string (nice catch!), add comments about
    do_hdmi_vsdb_modes() arguments and make it clearer that offset is
    relative to the end of the required fields of the HDMI VSDB
    (Ville Syrjälä)

v3: Fix 'Unknow' typo (Simon Farnsworth)
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Tested-by: NCancan Feng <cancan.feng@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67030Reviewed-by: NSimon Farnsworth <simon.farnsworth@onelan.co.uk>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NDave Airlie <airlied@gmail.com>