drm/connector: Add _DRM_MODE_CONNECTOR_COUNT

In order to add rust bindings for the various DRM_MODE_CONNECTOR_* type
IDs, we need a way to know the range of defined connector types - since
each type corresponds to an actual array entry in drm_connector_enum_list.

So, let's make this easy to figure out by adding a new
_DRM_MODE_CONNECTOR_COUNT macro that always matches the number of available
connector type IDs. Also, use that macro to define the size of
drm_connector_enum_list to ensure that builds fail if someone attempts to
add a new entry to drm_connector_enum_list without updating this macro.

Signed-off-by: Lyude Paul <lyude@redhat.com>