i965/fs: Determine partial writes based on the destination width

i965/fs: Determine partial writes based on the destination width

Now that we track both halves of a 16-wide vgrf, we no longer need to worry
about force_sechalf or force_uncompressed.  The only real issue is if the
destination is too small.

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