VIDEO: amba clcd: don't disable an already disabled clock

Fix the clock enable/disable tracking in the AMBA CLCD driver so
that the driver doesn't try to disable an already disabled clock,
thereby causing the clock (if shared) to become unbalanced.

This resolves a problem with CLCD on LPC32xx ARM platforms.

Reported-by: Kevin Wells <wellsk40@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>