Fix wrong assumptions in cea_for_each_detailed_block v2

The current logic misunderstands the spec about CEA 18byte descriptors.
First, the spec doesn't state "detailed timing descriptors" but "18 byte
descriptors", so any data record could be stored, mixed timings and
other data, just as in the standard EDID.
Second, the lower four bit of byte 3 of the CEA record do not contain
the number of descriptors, but "the total number of DTDs defining native
formats in the whole EDID [...], starting with the first DTD in the DTD
list (which starts in the base EDID block)." A device can of course
support non-native formats.

As such the number can't be used to determine n, and the existing code
will filter non-timing 18byte descriptors anyway.

Signed-off-by: Christian Schmidt <schmidt@digadd.de>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>