drm/i915: skip forcewake actions on forcewake-less uncore

We always call some of the setup/cleanup functions for forcewake, even
if the feature is not actually available. Skipping these operations if
forcewake is not available saves us some operations on older gens and
prepares us for having a forcewake-less display uncore.

v2: do not make suspend/resume functions forcewake-specific (Chris,
Tvrtko), use GEM_BUG_ON in internal forcewake-only functions (Tvrtko)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190620010021.20637-5-daniele.ceraolospurio@intel.com