i965: Only map the necessary buffer range in brw_prepare_indices

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i965/brw_draw_upload.c

@@ -690,11 +690,10 @@ static void brw_prepare_indices(struct brw_context *brw)
        */
        if ((get_size(index_buffer->type) - 1) & offset) {
            GLubyte *map = ctx->Driver.MapBufferRange(ctx,
-						     0,
-						     bufferobj->Size,
+						     offset,
+						     ib_size,
 						     GL_MAP_WRITE_BIT,
 						     bufferobj);
-           map += offset;
 
 	   intel_upload_data(&brw->intel, map, ib_size, ib_type_size,
 			     &bo, &offset);