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
Status Job ID Name Coverage
  Preparation
passed #54691
manifest

00:00:21

 
  Build
passed #54692
build fedora x86_64

00:02:57

 
  Full Builds
passed #54693
build android arm64 api28

00:02:45