xfree86: Add a xf86MonitorFindHDMIBlock()

xfree86: Add a xf86MonitorFindHDMIBlock()

The HDMI CEA vendor specific block has some interesting information,
such as the maximum TMDS dot clock.

v2: Don't parse CEA blocks with invalid offsets, remove spurious
    brackets (Chris Wilson)

v3: Fix the looping through the CEA data blocks, it had a typo using the
    wrong variable coming from the code it was ported from.
    Replace x << 16 + y << 8 + z by x << 16 | y << 8 | z
    (Chris Wilson)

v4: Remove the stray ';' at the end of "if (*end == 0)".
    (Dominik Behr on IRC)

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>