[platforms/drm] Pass ownership of gbm_device to Platform

Just like 4e7392b9075d78066218e55250e266fddaa03a7c:
the ownership of the gbm_device must be passed to the Platform as
the ownership of the EGLDisplay is also passed to the Platform and
we may not destroy the gbm_device for an EGLDisplay we are still using.

With this change I could restart the OpenGL compositor successfully
and switch from OpenGL 3.1 to OpenGL 2 without a crash or rendering
issues.