drm/i915/gen12: Fix HDC pipeline flush

HDC pipeline flush is bit on the first dword of
the PIPE_CONTROL, not the second. Make it so.

v2: function naming (Chris)

Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200506144734.29297-2-mika.kuoppala@linux.intel.com