i965/nir: double/dvec2 uniforms only need to be padded to a single vec4 slot

i965/nir: double/dvec2 uniforms only need to be padded to a single vec4 slot

max_vector_size is used in the vec4 backend to pad out the uniform
components to match a size that is a multiple of a vec4. Double and dvec2
uniforms only require a single vec4 slot, not two.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>