drm/i915: Inline intel_modeset_gem_init()

intel_modeset_gem_init() now only sets up the legacy overlay, so let's
remove the function and call the setup directly during driver load. This
should help us find a better point in the initialisation sequence for it
later.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110142634.10551-5-chris@chris-wilson.co.uk