gen{7,8}_pipeline: Always set ViewportXYClipTestEnable

gen{7,8}_pipeline: Always set ViewportXYClipTestEnable

For the following reasons, there is no behavioural change with this
commit: the ViewportXYClipTest function of the CLIP stage will continue
to be enabled outside of Meta (where disable_viewport is always false),
and the CLIP stage is turned off within Meta, so this function will
continue to be disabled in that case.

Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Kristian Høgsberg Kristensen <kristian.h.kristensen@intel.com>