drm/i915/gen9_lp: Fix DMC DC counter debugfs output

On GEN9 LP (BXT/GLK) DC6 is not supported, so don't print the counter
on those platforms. So far we did this on GLK too.

While at it warn if we forgot to adjust the printout properly for a
new platform. (Rodrigo)

Testcase: igt/pm_dc/dc6-dpms
Cc: Jyoti Yadav <jyoti.r.yadav@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031200220.11608-1-imre.deak@intel.com