media: v4l2-dv-timings: add v4l2_num_edid_blocks() helper

This new function determines how many blocks the EDID has.
Traditionally the number of extension blocks is read from
the EDID at offset 126, but this can be overridden by the
HDMI Forum EDID Extension Override Data Block. So check
that as well in this helper.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>