nir: Flip gl_SamplePosition in nir_lower_wpos_ytransform().

nir: Flip gl_SamplePosition in nir_lower_wpos_ytransform().

Assuming the hardware is set up to use a screen coordinate system
flipped vertically with respect to the GL's window coordinate system,
the SYSTEM_VALUE_SAMPLE_POS vector will also be flipped vertically
with respect to the value expected by the GL, so we need to give it
the same treatment as gl_FragCoord.  Fixes the following CTS tests on
i965:

 ES31-CTS.functional.shaders.multisample_interpolation.interpolate_at_offset.at_sample_position.default_framebuffer
 ES31-CTS.functional.shaders.sample_variables.sample_pos.correctness.default_framebuffer

when run with any multisample configuration, e.g. rgba8888d24s8ms4.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>