drm/i915/display: Share intel_connector_needs_modeset()

intel_connector_needs_modeset() will be used outside of
intel_display.c in a future patch so it would only be necessary to
remove the state and add the prototype to the header file.

But while at it, I simplified the arguments and moved it to a better
place intel_atomic.c.

No behavior changes intended here.

v3:
- removed digital from exported version of intel_connector_needs_modeset
- rollback connector to drm type

v4:
- Renamed new_connector_state to new_conn_state
- Going back to drm_connector_state in
intel_encoders_update_prepare/complete as we also have
intel_tv_connector_state

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191223010654.67037-1-jose.souza@intel.com