i965/fs: Do not set the size for zero-size uniforms

i965/fs: Do not set the size for zero-size uniforms

Zero sized uniforms can exist in the list, but they don't get get any space
allocated in prog_data->params or in the param_size array, so the size
should not be set for them.  This was previously fixed in:

commit: 781dc7c0e1f41502f18e07c0940af949a78d2792.

However,

commit: 259f7291de2387aa3ac5f856b39b7b934a1d8e7d

removed the fix.

Signed-off-by: Marta Lofstedt <marta.lofstedt@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>