intel/cs: Stop setting dispatch_grf_start_reg

Nothing ever reads it for compute shaders because it's always 1.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/intel/compiler/brw_compiler.h

@@ -745,7 +745,6 @@ struct brw_push_const_block {
 struct brw_cs_prog_data {
    struct brw_stage_prog_data base;
 
-   GLuint dispatch_grf_start_reg_16;
    unsigned local_size[3];
    unsigned simd_size;
    unsigned threads;