drm/vc4: Add a bitmap of branch targets during shader validation.

This isn't used yet, it's just a first step toward loop validation.
During the main parsing of instructions, we need to know when we hit a
new basic block so that we can reset validated state.

v2: Fix a stray semicolon after an if block.  (caught by kbuild test).

Signed-off-by: Eric Anholt <eric@anholt.net>