assembler: Replace struct direct_reg by struct brw_reg

More code simplification can be layered on top of that (by using some
brw_* helpers to create registers), that'd be for another commit.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>

assembler/gen4asm.h

@@ -66,14 +66,6 @@ typedef struct { \
 /* ensure nobody changes the size of struct brw_instruction */
 STRUCT_SIZE_ASSERT(brw_instruction, 16);
 
-/**
- * This structure is the internal representation of directly-addressed
- * registers in the parser.
- */
-struct direct_reg {
-	int reg_file, reg_nr, subreg_nr;
-};
-
 struct condition {
     	int cond;
 	int flag_reg_nr;