i965/fs: Print fs_reg::offset field consistently for all register files.

i965/fs: Print fs_reg::offset field consistently for all register files.

The offset printing code in fs_visitor::dump_instruction() was doing
things differently for sources and destinations and for each register
file -- In some cases it would be added to the base register number
fs_reg::nr, in other cases it would follow the base register separated
with a plus sign, in other cases (uniforms) it would do both (!).  The
sub-register offset was also being printed or not rather
inconsistently.  Fix it.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>