drm/i915: Rename and inline i915_gem_context_get()

i915_gem_context_get() is a very simple wrapper around idr_find(), so
simple that it would be smaller to do the lookup inline. Also we use the
verb 'lookup' to return a pointer from a handle, freeing 'get' to imply
obtaining a reference to the context.

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/1464098023-3294-3-git-send-email-chris@chris-wilson.co.uk