On VLV we depend on RC6 to save the GT render and media HW context
before going to the D3 state via RPM, so as a preparation for the
VLV RPM support (added in an upcoming patch) disable RPM if RC6 is
disabled.

There is probably a similar dependency on other platforms too, so for
safety require RC6 for those too. For these platforms (SNB, HSW, BDW)
this is then a possible fix.

v2:
- require RC6 for all RPM platforms, not just for VLV (Paulo, Daniel)
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>