drm/i915: Drop runtime-pm assert from vgpu io accessors

The "mmio" writes into vgpu registers are simple memory traps from the
guest into the host. We do not need to assert in the guest that the
device is awake for the io as we do not write to the device itself.

However, over time we have refactored all the mmio accessors with the
result that the vgpu reuses the gen2 accessors and so inherits the
assert for runtime-pm of the native device. The assert though has
actually been there since commit 3be0bf5a ("drm/i915: Create vGPU
specific MMIO operations to reduce traps").

References: 3be0bf5a ("drm/i915: Create vGPU specific MMIO operations to reduce traps")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Yan Zhao <yan.y.zhao@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200811092532.13753-1-chris@chris-wilson.co.uk