i965: Replace sizeof(struct gen7_sampler_state) with the size itself.

i965: Replace sizeof(struct gen7_sampler_state) with the size itself.

These are the last users of struct gen7_sampler_state.

v2: Use a local sampler_state_size variable, to help distinguish the
    various 16s (suggested by Topi Pohjolainen).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>