drm/edid: Check the number of detailed timing descriptors in the CEA ext block

CEA-861 says :
"d = offset for the byte following the reserved data block.
 If no data is provided in the reserved data block, then d=4.
 If no DTDs are provided, then d=0."

So let's not look for DTDs when d==0. In fact let's just make that
<4 since those values would just mean that he DTDs overlap the block
header. And let's also check that d isn't so big as to declare
the descriptors to live past the block end, although the code
does already survive that case as we'd just end up with a negative
number of descriptors and the loop would not do anything.

Cc: Allen Chen <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Acked-by: Alex Deucher <[email protected]>
Reviewed-by: Uma Shankar <[email protected]>

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