drm: zte: remove obsolete DRM Support for ZTE SoCs

Commit 89d4f98a ("ARM: remove zte zx platform") removes the config
ARCH_ZX. So, since then, the DRM Support for ZTE SoCs (config DRM_ZTE)
depends on this removed config ARCH_ZX and cannot be selected.

Fortunately, ./scripts/checkkconfigsymbols.py detects this and warns:

ARCH_ZX
Referencing files: drivers/gpu/drm/zte/Kconfig

So, remove this obsolete DRM support.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210819112253.16484-5-lukas.bulwahn@gmail.com