drm/i915: Wrap drm_gem_object_reference in i915_gem_object_get

Ultimately wraps kref_get(), so adopt its nomenclature for consistency
with other subsystems.

s/drm_gem_object_reference/i915_gem_object_get/

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469005202-9659-5-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469017917-15134-4-git-send-email-chris@chris-wilson.co.uk