The content of drm_global.{c,h} is obsolete.

v2: rebase on dropping TTM functionality
Signed-off-by: NThomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NJunwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>