drm/hyperv: Fix address space leak when Hyper-V DRM device is removed

In the Linux kernel, the following vulnerability has been resolved:

drm/hyperv: Fix address space leak when Hyper-V DRM device is removed

When a Hyper-V DRM device is probed, the driver allocates MMIO space for

the vram, and maps it cacheable. If the device removed, or in the error

path for device probing, the MMIO space is released but no unmap is done.

Consequently the kernel address space for the mapping is leaked.

Fix this by adding iounmap() calls in the device removal path, and in the

error path during device probing.