drm/i915/bios: fix off by one in parse_generic_dtd()

The "num_dtd" variable is the number of elements in the
generic_dtd->dtd[] array so the > needs to be >= to prevent reading one
element beyond the end of the array.

Fixes: 33ef6d4f

 ("drm/i915/vbt: Handle generic DTD block")
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191212091130.zf2g53njf5u24wk6@kili.mountain