msm: mdss: protect clk enable member with mdp lock

During irq_disable, mdp_lock is held and intr status is checked and
cleared if any. If a new irq is triggered from another CPU at the
same point, it would ideally be waiting on mdp_lock held by the other
CPU. And when the mdp_lock is released after clearing the irq, mdp_isr
is executed and at this point, clks might have been disabled. To avoid
it, protect the clk_ena variable with mdp_lock and also check for the
clk_ena status and skip irq handling when it is disabled.

Change-Id: Ic71d2b6f877ca3510a0d0fa593a8a0c17e93d8f3
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>