drm/i915/tgl: Gen12 render context size

Re-use Gen11 context size for now.

[ Lucas: this is a temporary enabling patch that needs to be confirmed:
         we need to check BSpec 46255 and recompute ]

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190817093902.2171-27-lucas.demarchi@intel.com