drm/amdgpu: fix rmmio iounmap skipped on device removal

amdgpu_pci_remove() calls drm_dev_unplug() before fini_sw(), so
drm_dev_enter() is already false there and the iounmap() guarded by it
is skipped. This .remove path runs on both hot-unplug and plain rmmod,
so the register BAR ioremap mapping leaks one instance per unload.

Unmap rmmio unconditionally (guard only on non-NULL) and drop the now
unused idx.

Fixes: 62d5f9f7110a ("drm/amdgpu: Unmap MMIO mappings when device is not unplugged")
Signed-off-by: Chengjun Yao <Chengjun.Yao@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit dd6f86a97260e5207d3329ad03aa89fdad61b1e6)
Cc: stable@vger.kernel.org