drm/displayid: allow data blocks with 0 payload length

The DisplayID specifications explicitly call out 0 as a valid payload
length for data blocks. The mere presence of a data block, or the
information coded in the block specific data (bits 7:3 in offset 1), may
be enough to convey the necessary information.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d562dff99ba7c92accb654a99b433bed471e8507.1617024940.git.jani.nikula@intel.com