drm/i915: clean up unused vgpu_read/write

Having converted the force_wake_get/_put routines for a vGPU to be no-op,
we can use the common mmio accessors and remove our specialised routines
that simply skipped the calls to control force_wake.

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1485408228-12932-1-git-send-email-weinan.z.li@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>