i965: Use scope operator to ensure brw_reg is interpreted as a type.

i965: Use scope operator to ensure brw_reg is interpreted as a type.

In the next patch, I make backend_reg's inheritance from brw_reg
private, which confuses clang when it sees the type "struct brw_reg" in
the derived class constructors, thinking it is referring to the
privately inherited brw_reg:

brw_fs.cpp:366:23: error: 'brw_reg' is a private member of 'brw_reg'
fs_reg::fs_reg(struct brw_reg reg) :
                      ^
brw_shader.h:39:22: note: constrained by private inheritance here
struct backend_reg : private brw_reg
                     ^~~~~~~~~~~~~~~
brw_reg.h:232:8: note: member is declared here
struct brw_reg {
       ^

Avoid this by marking brw_reg with the scope resolution operator.