drm/i915: Use the engine class to get the context size

drm/i915: Use the engine class to get the context size

Technically speaking, the context size is per engine class, not per
instance.

v2: Add MISSING_CASE (Tvrtko)

v3: Rebased

v4: Restore the interface back to hiding the class lookup (Chris)

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1491905472-16189-1-git-send-email-oscar.mateo@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>