drm/i915/gvt: remove unused to_gvt() and reduce includes

gvt.h has no need to include i915_drv.h once the unused to_gvt() has
been removed.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230926121904.499888-1-jani.nikula@intel.com