drm/amdgpu: Forbid runtime pm on driver unload

The PCI core calls pm_runtime_forbid() on device probe in pci_pm_init(),
making this the default state when amdgpu is loaded.
amdgpu_driver_load_kms() therefore calls pm_runtime_allow(), but there's
no pm_runtime_forbid() in amdgpu_driver_unload_kms() to balance it. Add
it so that we leave the device in the same state that we found it.

This isn't a bug, it's just good housekeeping. When amdgpu is first
loaded with runpm=1, then unloaded and loaded again with runpm=0,
pm_runtime_forbid() will be called from amdgpu_pmops_runtime_idle() or
amdgpu_pmops_runtime_suspend(), so the behaviour is correct. If there
ever is a third party driver for AMD cards, this commit avoids that it
has to clean up behind amdgpu.

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/ccd4f7208acbd7761364418fc34f7849acbb4597.1465392124.git.lukas@wunner.de