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

Add stubs for !I915. Not all the functions need to be stubbed.

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/930c8d332d425a54dae7af16aed26ec293f80afd.1694514689.git.jani.nikula@intel.com