i965/fs: Replace fs_reg::subreg_offset with fs_reg::offset expressed in bytes.

i965/fs: Replace fs_reg::subreg_offset with fs_reg::offset expressed in bytes.

The fs_reg::subreg_offset and ::offset fields are now redundant, the
sub-GRF offset can just be added to the single ::offset field
expressed in byte units.  The current subreg_offset value can be
recovered by applying the following rule: Replace each rvalue
reference of subreg_offset like 'x = r.subreg_offset' with 'x =
r.offset % reg_unit', and each lvalue reference like 'r.subreg_offset
= x' with 'r.offset = ROUND_DOWN_TO(r.offset, reg_unit) + x'.

For the same reason as in the previous patches, this doesn't attempt
to be particularly clever about simplifying the result in the interest
of keeping the rather lengthy patch as obvious as possible.  I'll come
back later to clean up any ugliness introduced here.

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