drm/ast: Support multiple outputs

Systems with AST graphics can have multiple output; typically VGA
plus some other port. Record detected output chips in a bitmask and
initialize each output on its own.

Assume a VGA output by default and use SIL164 and DP501 if available.
For ASTDP assume that it can run in parallel with VGA.

Tested on AST2100.

v3:
	* define a macro for each BIT(ast_tx_chip) (Patrik)
v2:
	* make VGA/SIL164/DP501 mutually exclusive

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Fixes: a59b0264 ("drm/ast: Initialize encoder and connector for VGA in helper function")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220607092008.22123-2-tzimmermann@suse.de