drm/gma500: Do not include <drm/drm_global.h>

The gma500 driver has no dependencies on drm_global.h. Remove the include
statement.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180817082138.26628-1-tzimmermann@suse.de