i915/gvt: Fix NULL pointer dereference in init_mmio_block_handlers

i915/gvt: Fix NULL pointer dereference in init_mmio_block_handlers

Fix following coccicheck error:
./drivers/gpu/drm/i915/gvt/handlers.c:2925:35-41: ERROR: block is NULL but dereferenced.

Use gvt->mmio.mmio_block instead of block to avoid NULL pointer
dereference when find_mmio_block returns NULL.

Fixes: 5cf89ca030a2 ("i915/gvt: Separate the MMIO tracking table from GVT-g")
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20220427115457.836729-1-wanjiabing@vivo.com
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>