drm/i915/gvt: Don't read ADPA_CRT_HOTPLUG_MONITOR from host

When host connects a crt screen, linux guest will detect two
screens: crt and dp. This is wrong as linux guest has only
one dp.

In order to avoid guest get host crt screen, we should set
ADPA_CRT_HOTPLUG_MONITOR to none. But MMIO_RO(PCH_ADPA) prevent
from that. So MMIO_DH should be used instead of MMIO_RO.

v2: Clear its staus to none at initialize, so guest don't
    get host crt.(Zhangyu)
v3: SKL doesn't have this register, limit it to pre_skl.(xiong)

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>