i965/fs: Rework GEN5 texturing code to use fs_reg and offset()

i965/fs: Rework GEN5 texturing code to use fs_reg and offset()

Now that offset() can properly handle MRF registers, we can use an MRF
fs_reg and let offset() handle incrementing it correctly for different
dispatch widths.  While this doesn't have any noticeable effect currently,
it does ensure that the destination register is 16-wide which will be
necessary later when we start detecting execution sizes based on source and
destination registers.

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