drm/i915/gvt: emulate gen9 dbuf ctl register access

drm/i915/gvt: emulate gen9 dbuf ctl register access

there is below call track at boot time when booting guest
with kabylake vgpu with specifal configuration and this try to fix it.

[drm:gen9_dbuf_enable [i915]] *ERROR* DBuf power enable timeout
------------[ cut here ]------------
WARNING: gen9_dc_off_power_well_enable+0x224/0x230 [i915]
Unexpected DBuf power power state (0x8000000a)
Hardware name: Red Hat KVM, BIOS 1.11.0-2.el7 04/01/2014
Call Trace:
 [<ffffffff99d24408>] dump_stack+0x19/0x1b
 [<ffffffff996926d8>] __warn+0xd8/0x100
 [<ffffffff9969275f>] warn_slowpath_fmt+0x5f/0x80
 [<ffffffffc07bbae4>] gen9_dc_off_power_well_enable+0x224/0x230 [i915]
 [<ffffffffc07ba9d2>] intel_power_well_enable+0x42/0x50 [i915]
 [<ffffffffc07baa6a>] __intel_display_power_get_domain+0x8a/0xb0 [i915]
 [<ffffffffc07bdb93>] intel_display_power_get+0x33/0x50 [i915]
 [<ffffffffc07bdf95>] intel_display_set_init_power+0x45/0x50 [i915]
 [<ffffffffc07be003>] intel_power_domains_init_hw+0x63/0x8a0 [i915]
 [<ffffffffc07995c3>] i915_driver_load+0xae3/0x1760 [i915]
 [<ffffffff99bd6580>] ? nvmem_register+0x500/0x500
 [<ffffffffc07a476c>] i915_pci_probe+0x2c/0x50 [i915]
 [<ffffffff9999cfea>] local_pci_probe+0x4a/0xb0
 [<ffffffff9999e729>] pci_device_probe+0x109/0x160
 [<ffffffff99a79aa5>] driver_probe_device+0xc5/0x3e0
 [<ffffffff99a79ea3>] __driver_attach+0x93/0xa0
 [<ffffffff99a79e10>] ? __device_attach+0x50/0x50
 [<ffffffff99a77645>] bus_for_each_dev+0x75/0xc0
 [<ffffffff99a7941e>] driver_attach+0x1e/0x20
 [<ffffffff99a78ec0>] bus_add_driver+0x200/0x2d0
 [<ffffffff99a7a534>] driver_register+0x64/0xf0
 [<ffffffff9999df65>] __pci_register_driver+0xa5/0xc0
 [<ffffffffc0929000>] ? 0xffffffffc0928fff
 [<ffffffffc0929059>] i915_init+0x59/0x5c [i915]
 [<ffffffff9960210a>] do_one_initcall+0xba/0x240
 [<ffffffff9971108c>] load_module+0x272c/0x2bc0
 [<ffffffff9997b990>] ? ddebug_proc_write+0xf0/0xf0
 [<ffffffff997115e5>] SyS_init_module+0xc5/0x110
 [<ffffffff99d36795>] system_call_fastpath+0x1c/0x21

Signed-off-by: Xiaolin Zhang <xiaolin.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>