drm/i915/gvt: Add missing forward decl of intel_vgpu for HDRTEST

./drivers/gpu/drm/i915/gvt/cmd_parser.h:53:44: error: ‘struct intel_vgpu’ declared inside parameter list will not be visible outside of this definition or declaration

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210121132558.2893-1-chris@chris-wilson.co.uk