r300/compiler: Move declaration before code.

r300/compiler: Move declaration before code.

Fixes these GCC warnings on linux-x86 build.
radeon_pair_translate.c: In function ‘set_pair_instruction’:
radeon_pair_translate.c:153: warning: ISO C90 forbids mixed declarations and code
radeon_pair_translate.c:170: warning: ISO C90 forbids mixed declarations and code
radeon_pair_translate.c: In function ‘rc_pair_translate’:
radeon_pair_translate.c:336: warning: ISO C90 forbids mixed declarations and code
radeon_pair_translate.c:341: warning: ISO C90 forbids mixed declarations and code