drm/i915: sanitize enable_rc6 option

Atm, an invalid enable_rc6 module option will be silently ignored, so
emit an info message about it. Doing an early sanitization we can also
reuse intel_enable_rc6() in a follow-up patch to see if RC6 is actually
enabled. Currently the caller would have to filter a non-zero return
value based on the platform we are running on. For example on VLV with
i915.enable_rc6 set to 2, RC6 won't be enabled but atm
intel_enable_rc6() would still return 2 in this case.

v2:
- simplify the platform check condition (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>