drm/i915: Move i915_gem_init_contexts() earlier

As the GEM global context setup is now independent of the GT state
(although GT does currently still depend upon the global
i915->kernel_context), we can move its init earlier, leaving the gt init
ready to be extracted.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191221200109.1202310-1-chris@chris-wilson.co.uk