drm/i915/gvt: cleanup usage for typed mmio reg vs. offset

We had previous hack that tried to accept either i915_reg_t or offset
value to access vGPU virtual/shadow regs which broke that purpose to
be type safe in context. This one trys to explicitly separate the usage
of typed mmio reg with real offset.

Old vgpu_vreg(offset) helper is used only for offset now with new
vgpu_vreg_t(reg) is used for i915_reg_t only. Convert left usage
of that to new helper.

Also fixed left KASAN warning issues caused by previous hack.

v2: rebase, fixup against recent mmio switch change

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