drm/radeon: Don't leak runtime pm ref on driver load

radeon_device_init() returns an error if either of the two calls to
radeon_init() fail. One level up in the call stack,
radeon_driver_load_kms() will then skip runtime pm initialization and
call radeon_driver_unload_kms(), which acquires a runtime pm ref that
is leaked.

Balance by releasing a runtime pm ref in the error path of
radeon_device_init().

Fixes: 10ebc0bc ("drm/radeon: add runtime PM support (v2)")
Cc: Dave Airlie <airlied@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/fa5bb977c1fe00474acedae5b03232dbf0b49410.1465392124.git.lukas@wunner.de