pan/va: Add missing valhall_enums dep to valhall_disasm

valhall_disasm compilation fails if the valhall_enums.h has
not be generated.

Fixes: 619566de ("pan/va: Generate header containing enums")
Closes: mesa/mesa#10553


Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <mesa/mesa!27524>
(cherry picked from commit 49069a12)