drm: remove unused drm_gem.h include

There's nothing GEM related in the file - the drm_gem.h include is not
needed.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515095118.2743122-2-emil.l.velikov@gmail.com