drm/i915/display: add I915 conditional build to vlv_dsi.h

Add stubs for !I915.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9f538824f15096b0c1444ee50d175a0a22d7bd5e.1694514689.git.jani.nikula@intel.com

drm/i915/dsi: split out vlv_dsi.h

Follow the convention of corresponding .h for .c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211122111504.223248-3-jani.nikula@intel.com