drm/i915: Rename i915_gem_context_reference/unreference()

As these are wrappers around kref_get/kref_put() it is preferable to
follow the naming convention and use the same verb get/put in our
wrapper names for manipulating a reference to the context.

s/i915_gem_context_reference/i915_gem_context_get/
s/i915_gem_context_unreference/i915_gem_context_put/

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: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469005202-9659-3-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1469017917-15134-2-git-send-email-chris@chris-wilson.co.uk