amd/common: Add some parentheses to silence warning.

amd/common: Add some parentheses to silence warning.

[1/59] Compiling C object 'src/amd/common/src@amd@common@@amd_common@sta/ac_nir_to_llvm.c.o'.
../mesa/src/amd/common/ac_nir_to_llvm.c: In function ‘get_inst_tessfactor_writemask’:
../mesa/src/amd/common/ac_nir_to_llvm.c:4089:32: warning: suggest parentheses around ‘+’ inside ‘<<’ [-Wparentheses]
   writemask = ((1 << num_comps + 1) - 1) << first_component;
                      ~~~~~~~~~~^~~
../mesa/src/amd/common/ac_nir_to_llvm.c:4091:33: warning: suggest parentheses around ‘+’ inside ‘<<’ [-Wparentheses]
   writemask = (((1 << num_comps + 1) - 1) << first_component) << 4;

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>