drm/hyperv: Fix device removal on Gen1 VMs

The Hyper-V DRM driver tries to free MMIO region on removing
the device regardless of VM type, while Gen1 VMs don't use MMIO
and hence causing the kernel to crash on a NULL pointer dereference.

Fix this by making deallocating MMIO only on Gen2 machines and implement
removal for Gen1

Fixes: 76c56a5a

 ("drm/hyperv: Add DRM driver for hyperv synthetic video device")

Signed-off-by: Mohammed Gamal <mgamal@redhat.com>
Reviewed-by: Deepak Rawat <drawat.floss@gmail.com>
Signed-off-by: Deepak Rawat <drawat.floss@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211119112900.300537-1-mgamal@redhat.com