gl/gbm: Add GST_GL_GBM_DRM_CONNECTOR environment variable

If multiple DRM connectors are connected, currently the first one is
picked. Improve this by adding an environment variable that allows for
choosing a connector by name. The connector name has been made so they
are compatible with modetest/modeprint DRM utilities.

Related to #490
3 jobs for 490-gbm-enhance-logging-and-add-environment-variable-for-selecting-drm-connector in 6 minutes and 4 seconds