drm: Initial KMS driver for AST (ASpeed Technologies) 2000 series (v2)

This is the initial driver for the Aspeed Technologies chips found in
servers. This driver supports the AST 2000, 2100, 2200, 2150 and 2300. It
doesn't support the AST11xx due to lack of hw to test it on, and them requiring
different codepaths.

This driver is intended to be used with xf86-video-modesetting in userspace.

This driver has a slightly different design than other KMS drivers, but
future server chips will probably share similiar setup. As these GPUs commonly
have low video RAM, it doesn't make sense to put the kms console in VRAM
always. This driver places the kms console into system RAM, and does dirty
updates to a copy in video RAM. When userspace sets a new scanout buffer,
it forcefully evicts the video RAM console, and X can create a framebuffer
that can use all of of video RAM.

This driver uses TTM but in a very simple fashion to control the eviction
to system RAM of the console, and multiple servers.

v2: add s/r support, fix Kconfig.

Signed-off-by: Dave Airlie <airlied@redhat.com>