anv/cmd_buffer: Reset state in cmd_buffer_destroy

anv/cmd_buffer: Reset state in cmd_buffer_destroy

This ensures that everything gets cleaned up properly. In particular,
it fixes a memory leak where we were leaking the push constants
structs.

Valgrind stats on
dEQP-VK.pipeline.push_constant.graphics_pipeline.range_size_128 :

Before:
HEAP SUMMARY:
    in use at exit: 2,467,513 bytes in 1,305 blocks
  total heap usage: 697,853 allocs, 696,530 frees, 138,466,600 bytes allocated

LEAK SUMMARY:
   definitely lost: 1,068 bytes in 11 blocks
   indirectly lost: 24,669 bytes in 412 blocks
     possibly lost: 0 bytes in 0 blocks
   still reachable: 2,441,776 bytes in 882 blocks
        suppressed: 0 bytes in 0 blocks

After:
HEAP SUMMARY:
    in use at exit: 2,467,381 bytes in 1,304 blocks
  total heap usage: 697,853 allocs, 696,531 frees, 138,466,600 bytes allocated

LEAK SUMMARY:
   definitely lost: 936 bytes in 10 blocks
   indirectly lost: 24,669 bytes in 412 blocks
     possibly lost: 0 bytes in 0 blocks
   still reachable: 2,441,776 bytes in 882 blocks
        suppressed: 0 bytes in 0 blocks

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: "17.2 17.1" <mesa-stable@lists.freedesktop.org>