drm/i915: WARN if !HAS_PC8 when enabling/disabling PC8

We already have some checks and shouldn't be reaching these places on
!HAS_PC8 platforms, but add a WARN,  just in case.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>