drm/i915/gen10+: use the SKL code for reading WM latencies

Gen 10 should use the exact same code as Gen 9, so change the check to
take this into consideration, and also assume that future platforms
will run this code.

Also add a MISSING_CASE(), just in case we do something wrong, instead
of silently failing.

Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170809205248.11917-1-rodrigo.vivi@intel.com