i965/fs: Silence warnings related to use of uninitialized values

i965/fs: Silence warnings related to use of uninitialized values

brw_fs.cpp: In function ‘const unsigned int* brw_compile_fs(const [...]
brw_fs.cpp:6093:64: warning: ‘simd16_grf_start’ may be used uninitialized [...]
       prog_data->base.dispatch_grf_start_reg = simd16_grf_start;

brw_fs.cpp:5996:29: note: ‘simd16_grf_start’ was declared here
    uint8_t simd8_grf_start, simd16_grf_start;

brw_fs.cpp:6094:52: warning: ‘simd16_grf_used’ may be used uninitialized [...]
       prog_data->reg_blocks_0 = brw_register_blocks(simd16_grf_used);

brw_fs.cpp:5997:29: note: ‘simd16_grf_used’ was declared here
    unsigned simd8_grf_used, simd16_grf_used;

(and more)

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>