batch: Specify number of relocations to accommodate

Since relocations are variable size, depending upon generation, it is
easier to handle the resizing of the batch request inside the
BEGIN_BATCH macro. This still leaves us with having to resize commands
in a few places - which still need adaption for gen8+.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

tests/gem_cs_prefetch.c

@@ -134,7 +134,7 @@ igt_simple_main
 
 		/* copy the sample batch with the gpu to the new one, so that we
 		 * also test the unmappable part of the gtt. */
-		BLIT_COPY_BATCH_START(batch->devid, 0);
+		BLIT_COPY_BATCH_START(0);
 		OUT_BATCH((3 << 24) | /* 32 bits */
 			  (0xcc << 16) | /* copy ROP */
 			  4096);