KMS is no longer a platform plugin so the relevant leftover bits are
now removed.

As the introduction of the EGLDevice-based backend for eglfs shows,
using DRM/KMS is not tied to GBM, separate buffer management
approaches, like EGLStreams, work fine as well. Therefore separate KMS
from GBM and remove the EGL and GLES dependency in the tests - this
way there is nothing preventing us from using GBM without GL for
example.

Change-Id: Id7ebe172b44b315f9a637892237d2bb62d99aed2
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Louai Al-Khanji <louai.al-khanji@theqtcompany.com>