drm/amdgpu: Fix driver reloading failure

SRIOV doesn't implement PMC capability of PCIe, so it can't update
power state by reading PMC register.

Currently, amdgpu driver doesn't disable pci device when removing
driver, the enable_cnt of pci device will not be decrease to 0.
When reloading driver, pci_enable_device will do nothing as
enable_cnt is not zero. And power state will not be updated as PMC
is not support.
So current_state of pci device is not D0 state and pci_enable_msi
return fail.

Add pci_disable_device when remmoving driver to fix the issue.

Signed-off-by: Xiangliang.Yu <Xiangliang.Yu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>