drm: Remove drm_global.{c,h} v2

The content of drm_global.{c,h} is obsolete.

v2: rebase on dropping TTM functionality

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>