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

If an error occurs in amdgpu_device_init() after adev->rmmio has been
set, its caller amdgpu_driver_load_kms() will skip runtime pm
initialization and call amdgpu_driver_unload_kms(), which acquires a
runtime pm ref that is leaked.

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

Fixes: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
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/9a53449865759d7499a439ca2776093ee117b1eb.1465392124.git.lukas@wunner.de