drm/i915: make intel_sdvo_regs.h self-contained

Ensure intel_sdvo_regs.h is self-contained and remains that way.

v2:
- include <linux/compiler.h> for __packed (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613100818.24800-1-jani.nikula@intel.com