drm/i915: Partition the fence registers for vGPU in i915 driver

With Intel GVT-g, the fence registers are partitioned by multiple
vGPU instances in different VMs. Routine i915_gem_load() is modified
to reset the num_fence_regs, when the driver detects it's running in
a VM. Accesses to the fence registers from vGPU will be trapped and
remapped by the host side. And the allocated fence number is provided
in PV INFO page structure. By now, the value of fence number is fixed,
but in the future we can relax this limitation, to allocate the fence
registers dynamically from host side.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Eddie Dong <eddie.dong@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>