Move display power related members under drm_i915_private display
sub-struct.

Arguably chv_phy_control and chv_phy_assert could be placed in a phy
substruct, but they are only used in the power code.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/57bfa82f6fe85a775f80c398b2a7dff77b9452b0.1661779055.git.jani.nikula@intel.com