freedreno/registers: Fix nameless fields

freedreno/registers: Fix nameless fields

Originally if we had an anonymous field (ie. field declared as part of
the register definition itself) the name in the generated field struct
would include the gen prefix (ie. .a6xx_rb_stencil_buffer_pitch), but
this doesn't work for variants because the variant regs would have
different gen prefixes.  Fix this by using reg name instead of the
full_name.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21846>