zink: fix -Warray-bounds warning

Seems rast_bits is too narrow on windows:

string.h:202:10: warning: ‘__builtin___memcpy_chk’ forming offset [4, 7] is out of the bounds [0, 4] of object ‘rast_bits’ with type ‘uint32_t’ {aka ‘unsigned int’} [-Warray-bounds]
  202 |   return __builtin___memcpy_chk(__dst, __src, __n, __mingw_bos(__dst, 0));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/gallium/drivers/zink/zink_state.c: In function ‘zink_bind_rasterizer_state’:
../src/gallium/drivers/zink/zink_state.c:586:16: note: ‘rast_bits’ declared here

Fixes: 9c5a2ab6

cc @zmike