aspeed-kernel: ast2400: Remove DRM and unused FSI drivers

Disable the SBE FIFO and related drivers, as they are Power9 only.

Disable the DRM framework and the ASPEED_GFX driver, as this driver has
not been tested on the ast2400.

Both of these changes are to reduce the size of the kernel binary, which
is currently 56136 bytes (55kB) over size.

Change-Id: Ice88b7ca9062c2dc4578a96ee5431c265fe75495
Signed-off-by: Joel Stanley <joel@jms.id.au>