i965/fs: Make null_reg_* const members of fs_visitor instead of globals

i965/fs: Make null_reg_* const members of fs_visitor instead of globals

We also set the register width equal to the dispatch width.  Right now,
this is effectively a no-op since we don't do anything with it.  However,
it will be important once we add an actual width field to fs_reg.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>