gbm: Enhance logging and add environment variable for selecting DRM connector

Submitted by Carlos Rafael Giani

Link to original bug (#797246)

Description

Currently, the first connected DRM connector is selected. This may not always be the right one. For this reason, add a GST_GL_GBM_DRM_CONNECTOR environment variable that allows for selecting a connector by name.

Also improve GBM logging by always logging all DRM connectors, modes, encoders, even if some have already been selected.

cc @ndufresne