drm/i915/gvt: declare gvt as i915's soft dependency

This helps initramfs builder and other tools to know the full dependencies
of i915 and have gvt module loaded with i915.

v2: add condition and change to pre-dependency (Chris)
v3: move declaration to gvt.c. (Chris)
v4: remove xengt (Zhenyu)

Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>