drm_hwcomposer: support more connector types

drm_hwcomposer: support more connector types

according to the drm_connector_enum_list defined
in the kernel file drivers/gpu/drm/drm_connector.c

Otherwise for the new connector type, the connector
name will be "None" to be printed  in the logcat, like this:
    12-13 05:38:06.263   383   383 I hwc-backend: Backend 'generic' for 'None' and driver 'omapdrm' was successfully set
with this change, it will be fixed with the connector type like this:
    12-13 06:01:57.672   342   342 I hwc-backend: Backend 'generic' for 'DPI-1' and driver 'omapdrm' was successfully set

Also updated the internal() and external() functions
to make the newly added SPI and USB are valid types

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>