i965/fs: Rename offset to tex_offset to avoid shadowing offset().

i965/fs: Rename offset to tex_offset to avoid shadowing offset().

fs_visitor::nir_emit_texture() created an fs_reg variable called offset,
which shadowed the offset() helper function in brw_ir_fs.h.

Rename the variable to tex_offset so we can still call offset().

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>