drm/i915/gvt: Rename intel_vgpu_{init, clean}_gvt_context()

To move workload related functions into scheduler.c, an expected way is
to collect all the init/clean functions related to vGPU workload
submission into fewer functions.

Rename intel_vgpu_{init, clean}_gvt_context() for above usage in future.

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>