i965/fs: Replace fs_inst::regs_written with ::size_written field in bytes.

i965/fs: Replace fs_inst::regs_written with ::size_written field in bytes.

The previous regs_written field can be recovered by rewriting each
rvalue reference of regs_written like 'x = i.regs_written' to 'x =
DIV_ROUND_UP(i.size_written, reg_unit)', and each lvalue reference
like 'i.regs_written = x' to 'i.size_written = x * reg_unit'.

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>