drm/i915: skip display initialization when there is no display

Display features should not be initialized or de-initialized when there
is no display. Skip modeset initialization, output setup, plane, crtc,
encoder, connector registration, display cdclk and rawclk
initialization, display core initialization, etc.

Skip the functionality at as high level as possible, and remove any
redundant checks. If the functionality is conditional to *other* display
checks, do not add more. If the un-initialization has checks for
initialization, do not add more.

We explicitly do not care about any GMCH/VLV/CHV code paths, as they've
always had and will have display.

Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210408203150.237947-3-jose.souza@intel.com