drm/ast: Protect concurrent access to I/O registers with lock

Add a mutex lock to protect concurrent access to I/O registers
against each other. This happens between invocation of commit-
tail functions and get-mode operations. Both with use the CRTC
index register AST_IO_CRTC_PORT. Concurrent access can lead to
failed mode-setting operations.

v2:
	* fix typo in commit description (Jocelyn)

Signed-off-by: Thomas Zimmermann <[email protected]>
Reviewed-by: Jocelyn Falempe <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Reported-by: KuoHsiang Chou <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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