i965/vec4: Use the uniform count from nir_assign_var_locations

i965/vec4: Use the uniform count from nir_assign_var_locations

Previously, we were counting up uniforms as we set them up.  However, this
count should be exactly identical to shader->num_uniforms provided by
nir_assign_var_locations.  (If it's not, we're in trouble anyway because
that means that locations don't match up.)  This matches what the fs
backend is already doing.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>