drm: DRM_KMB_DISPLAY should depend on ARCH_KEEMBAY

The Intel Keem Bay display controller is only present on Intel Keem Bay
SoCs.  Hence add a dependency on ARCH_KEEMBAY, to prevent asking the
user about this driver when configuring a kernel without Intel Keem Bay
platform support.

Note that:
  1. The dependency on ARM is dropped, as Keem Bay SoCs are only
     supported in arm64 kernel builds,
  2. The dependencies on OF and COMMON_CLK can be dropped for
     compile-testing, as the driver builds fine regardless.

Fixes: ed794057 ("drm/kmb: Build files for KeemBay Display driver")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201110144350.3279147-1-geert+renesas@glider.be