drm/armada: fix colorkey mode property

[ Upstream commit d378859a667edc99e3473704847698cae97ca2b1 ]

The colorkey mode property was not correctly disabling the colorkeying
when "disabled" mode was selected.  Arrange for this to work as one
would expect.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>