drm/i915/wopcm: update default size for gen11+

The size has been increased to 2MB starting from Gen11. GuC and HuC FWs
fit in 1MB so we were fine even with the legacy define, but let's still
move to the correct one before the blobs grow to avoid being caught off
guard in the future.

v2: return early if the platform doesn't have GuC, fix nits (Michal)

Bspec: 12690
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
[ickle: use SZ consistently]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190606224225.14287-2-daniele.ceraolospurio@intel.com