drm/armada: fix irq handling

[ Upstream commit 92298c1c ]

Add the missing locks to the IRQ enable/disable paths, and fix a comment
in the interrupt handler: reading the ISR clears down the status bits,
but does not reset the interrupt so it can signal again.  That seems to
require a write.

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>