drm/ast: Distinguish among chip generations

ASpeed distinguishes among various generations of the AST graphics
chipset with various models. [1] The most-recent model AST 2600 is
of the 7th generation, the AST 2500 is of the 6th generation, and so
on.

The ast driver simply picks one of the models as representative for
the whole generation. In several places, individual models of the
same generation need to be handled differently, which then requires
additional code for detecting the model.

Introduce different generations of the Aspeed chipset. In the source
code, refer to the generation instead of the representation model where
possible. The few places that require per-model handling are now clearly
marked.

In the enum ast_chip, we arrange each model's value such that it
encodes the generation. This allows for an easy test. The actual values
are ordered, but not of interest to the driver.

v2:
	* use __ast_gen_is_eq() (Jingfeng)

Signed-off-by: Thomas Zimmermann <[email protected]>
Link: https://web.archive.org/web/20141007093258/http://www.aspeedtech.com/products.php?fPath=20 # 1
Reviewed-by: Jocelyn Falempe <[email protected]>
Tested-by: Jocelyn Falempe <[email protected]> # AST2600
Reviewed-by: Sui Jingfeng <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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