panfrost: Rectify doubleplusungood extended branch
Midgard features "compact branches" and "extended branches", i.e.
corresponds to short jumps and far jumps. The form of the extended
branch was previously incorrect in the ISA headers; this patch corrects
it and updates the disassembler (simultaneous to preserve
bisectability).
Additionally, we fix some a corner case in the disassembly of extended
branches, and we now prefix extended branches with "brx", to visually
differentiate from compact branches prefixed with "br".
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>