drm/i915: Do not export RC6p and RC6pp if they don't exist

Avoid to expose RC6 and RC6pp to the platforms that doesn't support it.
So powertop can be changed to show RC6p and RC6pp only on the platforms
they are available.

v2: Simplify by merging RC6p and RC6pp groups and respect the spec that
mentions deep and deepest RC6 on SNB and IVB although they keep disabled
by default.

v3: Remove unecessary space.

v4: RC6p and RC6pp is only for SNB and IVB; unify debug msg and use
    has_rc6p() on sanitize options instead of is gen 6 and ivb.

v5: yet another fix on has_rc6p macro. final is_gen6 or is_ivb! To make sure
    we are excluding hsw and baytrail.

References: https://bugs.freedesktop.org/show_bug.cgi?id=84524


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