drm/i915: Merge legacy+execlists context structs

struct intel_context contains two substructs, one for the legacy RCS and
one for every execlists engine. Since legacy RCS is a subset of the
execlists engine support, just combine the two substructs.

v2: Only pin the default context for legacy mode (the object only exists
for legacy, but adding i915.enable_execlists provides symmetry with the
cleanup functions).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464098023-3294-8-git-send-email-chris@chris-wilson.co.uk