drm/nouveau: Runtime suspend despite HDA being unbound

Commit 5addcf0a ("nouveau: add runtime PM support (v0.9)") prevents
runtime suspend of the GPU if its integrated HDA controller is not bound
to a driver.  The rationale appears to be that probing the HDA fails if
the GPU is in D3cold.

However we now use a device link to ensure that the GPU is runtime
resumed while the HDA controller is probed, rendering this safety
measure obsolete.  Remove it.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Tested-by: Denis Lisov <dennis.lissov@gmail.com>       # Nvidia Optimus
Tested-by: Peter Wu <peter@lekensteyn.nl>              # Nvidia Optimus
Tested-by: Lukas Wunner <lukas@wunner.de>              # MacBook Pro
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://patchwork.freedesktop.org/patch/msgid/77e0ab74f3377ea9b6acf8fab624acfb4f7dbeca.1520068884.git.lukas@wunner.de