drm: Add privacy-screen class (v4)

On some new laptops the LCD panel has a builtin electronic privacy-screen.
We want to export this functionality as a property on the drm connector
object. But often this functionality is not exposed on the GPU but on some
other (ACPI) device.

This commit adds a privacy-screen class allowing the driver for these
other devices to register themselves as a privacy-screen provider; and
allowing the drm/kms code to get a privacy-screen provider associated
with a specific GPU/connector combo.

Changes in v2:
- Make CONFIG_DRM_PRIVACY_SCREEN a bool which controls if the drm_privacy
  code gets built as part of the main drm module rather then making it
  a tristate which builds its own module.
- Add a #if IS_ENABLED(CONFIG_DRM_PRIVACY_SCREEN) check to
  drm_privacy_screen_consumer.h and define stubs when the check fails.
  Together these 2 changes fix several dependency issues.
- Remove module related code now that this is part of the main drm.ko
- Use drm_class as class for the privacy-screen devices instead of
  adding a separate class for this

Changes in v3:
- Make the static inline drm_privacy_screen_get_state() stub set sw_state
  and hw_state to PRIVACY_SCREEN_DISABLED to squelch an uninitialized
  variable warning when CONFIG_DRM_PRIVICAY_SCREEN is not set

Changes in v4:
- Make drm_privacy_screen_set_sw_state() skip calling out to the hw if
  hw_state == new_sw_state

Reviewed-by: Emil Velikov <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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