drm/i915: dynamically allocate forcewake domains

We'd like to introduce a display uncore with no forcewake domains, so
let's avoid wasting memory and be ready to allocate only what we need.
Even without multiple uncore, we still don't need all the domains on all
gens.

v2: avoid hidden control flow, improve checks (Tvrtko), fix IVB special
case, add failure injection point

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
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/20190620010021.20637-6-daniele.ceraolospurio@intel.com