disp: msm: sde: avoid sde irq enable or disable when sde irq not active

During the sde_irq_update, the sde irq may not be active, the enable
or disable will be nonsence. set the irq_num init value to negtive, if
this value change, means sde irq_num has been initial.

Change-Id: I7b23c662ccc9db0b38550897adb939305d941e6a
Signed-off-by: longzhao <longzhao@codeaurora.org>