kernel: move DRM_DEBUG_* symbols to generic config

While working on a new target (meson), the kernel build failed due to
missing DRM_DEBUG_MM_SELFTEST symbol. This can potentially happen on all
targets that enable DRM drivers in the kernel config or via kmod
packages, so add it to the generic config and remove it from x86
subtarget configs, together with DRM_DEBUG_MM.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>